[MaxWilson]

It sounds like you have too few armies to defend your territory. The tips from Baalz and thejeff are very good (for your case you might especially benefit from building several forts so you're not chasing invading armies around the map), but it also sounds like you're trying to defend your borders with maybe a couple of big armies while the AIs nibble you to death. Once you develop a feel for exactly how much force is needed to defeat an enemy army you'll feel more comfortable splitting your armies into enough pieces that they can deal with several invaders at the same time. Don't underestimate the value of mages. In the right circumstances, 6 regular troops and 1 mage can defeat 70 invaders, and PD + a mage helps even more. Of course this depends upon what spells you've researched; toward the beginning of the game mages aren't as useful in battle, but once you get Iron Warriors, Blade Wind, Body Ethereal, Magma Eruption even PD can become quite tough. By the way, about PD: don't overlook the effect of morale. Spread friendly dominion where possible for the morale boost, and even a lowly H2 priest can cast Sermon of Courage which can tip the balance with PD quite frequently.

-Max