I use talcum powder. The liquids suggested for reducing the friction of the tire on the rim as you're trying to lever the last bit over tend to end up drying with the undesirable side effect of sticking the tube to the inside of the tire. Talcum powder will reduce the friction and keep the tube and tire from sticking to each other. Particularly in the MTB community, sticking of the 2 and the friction created whenever the tire flexes is believed to be an eventual cause of tube failure. On a MTB, there is far more relative movement between the 2 and flexing of the tire than a road bike, but the idea is still the same. The generous dusting of the tube prior to installation is all that's required. enough will end up on the rim and inside of the tire to do the job. Wipe off the rim with a wet cloth when you're done. As pointed out above, there is also some technique to learn.