Good handicaps for Descent
My experience with Descent is that some maps are balanced towards the Heroes, and others towards the Overlord.

If you still find the heroes are losing a lot on most of the maps, in my opinion, your best bet is to pick up The Tomb of Ice expansion. It adds Feat cards for the heroes which are quite powerful, especially when you add them to the base maps.

If buying an expansion is not an option for you, there are many, many ways to rebalance the game. Some examples could include: Reduce the Overlord's threat by 1/turn, give the heroes more starting coins, allow the heroes to pick their starting characters or feats, use the monster cards designed for 1 fewer player etc.

Some of these are bigger balance shifts than others, but if you play around with them a little, you'll probably find something that helps.

Finally, it's possible that your hero team just isn't playing as well as the Overlord. This forum has pages of strategy for the heroes and Overlord. Why not get the hero team to have a look? http://boardgamegeek.com/forum/429/descent-journeys-in-the-dark/strategy