I've always preferred the sound of a good shotgun mic over a lav. But, sometimes the framing doesn't allow for a shotgun mic. If you need to use lavs here are a few tips for you:

*Having the lav exposed and using a clip will definitely net better sound than placing it beneath clothing. *If the lav has to be hidden use a vampire clip on one side and a gaff tape triangle on the other. That will help reduce clothing noise. It won't eliminate clothing sound, it will simply reduce it.
*Use an omni lav, not a cardioid or other direction lav and place it somewhere on the chest, brim of a hat, etc.
*If the actor is stationary you might be able to place the mic on an object rather than on their body. For example, if the actor is standing in a door way you could place the lav on the door jam instead of on them. You'll get great sound, no clothing rustle, and the mic can easily be hidden. *When hiding the lav underneath clothing it helps to place it between two shirts as opposed to a shirt and the actors skin. If the actor is only wearing one shirt use a gaff tape triangle on the side with the shirt and hypoallergenic surgical tape on the skin side. *In spite of what some people think a lav does NOT eliminate background noise! I've had more than one director turn to me and say "just put a lav on the actor and then we won't hear all of the planes/trains/automobiles in the background." I then get to spend a few minutes educating them on signal to noise ratio and the way microphones actually work.

And on a separate note. If your choices are between Protools on a laptop or a DAT go with Protools. There are a million reasons to avoid DAT machines in this day and age.

Also, all wireless units are not created equally. A Lectrosonics SMQV transmitter with a Lectro 411 receiver sounds good. A Sennheiser G2 Tx/Rx do not sound good, even with the same capsules. I recently did a thorough comparison of a Lectro SMQV/411, Sennheiser 2000 series, Sennheiser G2, and Lectro UM400a/401. All transmitters had Sanken COS-11 capsules. The Lectrosonics and Sennheiser 2000 series all had the same audio quality. There were some differences in range and how the signal dropped out but that was about it. The Sennheiser G2 series was dramatically lower in quality in comparison to all the other Tx/Rx combos. Just an FYI.

This can vary for different actors. Assuming your microphones are omnidirectional (I wouldn't use anything else), you have a number of options. I generally, stick DPA's in the centre of the actors' chest, about six inches below the chin, using a Rycote sticky and a DPA disk. That is a good starting point. Sticking to skin minimises rustle and gives a little more presence to the tone, especially if the actor doesn't have a strong voice. If the artist is wearing a neck tie, feeding the wire inside the shirt, into the back of the tie and having the end of the microphone just inside the knot of the tie sticking out of bottom of the knot. If a female artist is wearing a low cut top, a microphone on the centre of her bra at the front is a good starting point. Don't just dive in there, ask her to place the microphone where you want it.

You're going to need some tape to stick the wire, and possibly microphone in place. Ask the costume department for some of their tape if possible, if you start using gaffer tape on costumes and messing up their work by leaving tape residue everywhere (even inside the costumes) they won't be happy. Use their tape, if it leaves marks, costume bought the wrong tape. Plus, Costume Dept will have double sided tape, which works great for keeping the wire in place, when used as single side tape. (I.E. Don't peel the back off.) Collect all access wire together and tape this up, with the transmitter. Remember to leave enough slack wire for the actor to move freely.

If the artist is wearing a jacket with an inside pocket, that is usually a good place for the transmitter to go. If the transmitter has a clip, put the transmitter inside the waist of the skirt/trousers on the artist's back, tucking the shirt OVER the transmitter. If you don't have a clip on the transmitter, use an elasticated belt, inside the shirt. You basically need to put the transmitter somewhere which is convenient for the artist and Costume Dept. It shouldn't be noticeable on camera. If a female artist is wearing a fitted dress, you'll probably need an elasticated thigh belt to velcro the transmitter to on the inside leg. For me, the artist normally puts this on in costume and arrives on set with the microphone wire lose out of the top of the dress. Just position the microphone as above and ask costume to tuck the access wire where is convenient for them.

You'll have more problems with actors who are softly spoken, or who don't project very well. Generally, actors don't mind if you ask them for more level in their performance. Clear this with the director forst though, but they should be fine with it. Use the rehearsal to listen to the microphone and decide if it needs moving.

I wouldn't suggest putting microphones in hats or hair if you aren't experienced at working with lapel microphones, but for interior car scenes (with the windows closed), you could try the sun visor technique - or anywhere which is out of shot and sounds good in there.

Be sure to use fresh cells as often as possible, no one wants to wait around for sound to change a battery. As mentioned earlier, always, always boom the scene (where possible) even if you're using radio microphones. Lastly, if you've tried everything and the microphone still isn't sounding ideal, then that is as good as it will get. If you've had a boom microphone on the scene, you should have enough tight dialogue from the tighter shots to cover your back. That should save the scene from needing ADR. Which is what the Line Producer will be pleased with - don't forget to mention it when you're discussing your next contract. :)

You'll get some great answers on this group. But here's where all of the pro production sound mixers hang out. A lot of seasoned experts on this group:

http://groups.google.com/group/rec.arts.movies.production.sound/topics

Generally lavs are pinned on the chest around the bottom of the ribcage. Other things to consider: try putting them in the actors hair, on the brim of a hat, on the top window visor of a car when doing car interiors, hiding them off of the actor in a prop such as a plant etc.. Whatever you do, avoid putting them under clothing as it creates what I like to call an "audio perfect storm": a muffled voice with a bunch of cloth scraping on top of it. Very hard to remove cloth from dialog, especially if you have to add presence EQ to make the dialog more intelligible.

I generally dislike the sound of lavs as they sound too dry, chesty, and unnatural. Whatever you do, don't ever just rely on them during a shot. Always boom.

Hope this helps.

In addition to Ryan's incredible advice, a few more tips from my experience:

• Don't cheat too high up the chest; you'll get attenuation and odd tones if the chin blocks the mic capsule.
• I love Dr. Scholl's moleskin for adhering and wrapping the mic itself, and either Topstick (wig/toupee tape) or 3M Propore medical tape for securing the cables to the talent and/or wardrobe.
• Always form a loop on each end of the mic cable for strain relief, and tape down the loop nearest the mic.
• I've also used inside-out loops of gaffer's tape made into triangles, like Inigo's suggestion, but I've found that (at least in the US) what we call duct tape will leave a ton of residue on wardrobe. Gaffer's tape is easier to handle and remove. (Paper tape, the type used for on-set spiking and setting of marks, isn't quite sticky enough.)
• Every situation is different! Don't hone one technique - always keep experimenting! Some tricks are better than others in some situations.

A colleague once showed me a practice that has been working well for hidding omnis like MKE-2 between clothes. Cut 2 pieces of gaffer tape (about 6cm long) and make one cylinder with each, with the glue side outside. Now carefully stick this two cylinders in parallel with the last 5-6 centimeters of the cable alligned between them. If you look from the top side the two pieces of tape should look like an "8" with the capsule appearing in the center. This way you can attach it between two clothes avoiding friction noises.

Be very careful with scarfs or foulards since they will attenuate the higher frequencies.

Necklaces are a potential problem; always check that there is no way for them to hit the capsule.

Edit: When I said "duct" I meant "gaffer", as NoiseJockey suggests.