Hi, I've had a fair bit of experience with this. If you can get a hold of headset mics instead, they're a million times better, but you can still get great results with lavs!
Could you post the actual model numbers of the mics? That would be very helpful.
As far as general pointers go, I usually clip the mic directly to the persons collar if they're wearing a T-shirt, high up on their lapel if they're wearing a jacket, or between the first and second buttons if they're wearing a shirt. I've seen them clipped to people's shirt collars, but IME they end up knocking it with their chin.
As for positioning, it depends on the mic. If the lav has an 'omni' pickup pattern, it really doesn't matter about the orientation! I usually point them downward, because it reduces the chances of getting breath noise down the mic. If the lav is cardioid, then you need to have the 'front' of the mic pointing straight up, towards the performers head.
The other thing to bear in mind about positioning, is that you might be able to learn a thing or two from the performers! If they do a lot of stage work, then they'll have been mic'd up before, and may well be able to share useful info.
You will find that feedback is your enemy here.
If you have access to a 32-band graphic EQ, it's very worth 'ringing out' the mics. Set them up in a group (or Bus) and use the 'group insert' to run all the mics through a single graphic. Leave them on the front of the stage (they don't need to be attatched to people, just put 'em on the stage) and gradually increase the level until you get feedback, then simply notch the frequency that's feeding.
Trial and error is fine if you don't find the offending frequency straight away. If you cut a fader and it doesn't solve the problem, remember to slide it back up to the middle! You want to do as little as possible with the EQ, just using it to remove the most 'ringy' frequencies.