Sometimes it can be very hard. But I've found that "building slack" into the tire usually works.

Basically you hold the tire with both hands like a big driving wheel and rock the tire all the way around. Then you try to push it in. If it doesn't work right away, do it again.

It's well described here: http://www.teamestrogen.com/content.ep?file=asa_levers

I will use levers to pull tires off quickly but never to put them back on.

This being said, some wheels can be really, really tough, campi wheels have a really bad reputation for that.