Going through some old emails I came across this and it is really good info!
When you buy a new tire, put a liberal amount of baby powder in the inside surface of the tire and spread it around, to cover all of that inside surface. Then shake the remaining loose powder out. In fact it's a good idea to keep your inner tubes dusted with baby powder all the time too. It lubricates the tube in the tire, it prevents the tube from sticking to the inside of the tire and prevents the small particles of debris that end up in the space between the tire and tubes from becoming embedded in one place in the tube.
NONE of this is true for tubeless clinchers or tubular tires.
With brand new tires that are proving tight there are a couple of tricks:
1. Warm the tire in the sun.
2. Put your foot in one side and pull the other as hard as you can. This worked better in the era of steel beads, since kevlar is less elastic.
3. Put the tire on the wheel with levers, but DON'T put the tube in yet ! Stretch the tire without the risk of pinching the tube. Once you've stretched the tire then you can put the tub in with less risk of a pinch.
4. Once the tube is full inserted into the tire, and at least half the tire is on the rim starting at the top, grip the tire and pull it as hard as you can down toward the open part of the rim. Move 6 inches down and do it again. And then again. This will gather any "slack" and put it where you need it, at the location of that last terrible 6 inches before the tire snaps on .
5. You should do this anyway, but especially with a new and or difficult tire. Once the tire is on pump it up just a little and then pinch the sidewalls together to expose the inside of the rim and check that the tube is not caught. Do this all the way around. If the tube is caught and you don't free it you will get a pinch flat before too long, if not immediately when you fully inflate the tire.
Courtesy Matt Sparks