How do you dwarf players combat the excellant movement of the other races? Do you just slide over the field, sucking up damage along the way and hope to make it?

Depends on the type of army.
But it is mostly due ro high toughness, strong armour and stubborness

The answer to my own question would have to be(maybe) to use cover to your advantage. Use the tree line to hide behind, and wait behind that building for a turn.

Weel like i said it depends.
for example if you play a shooty army you dont have to march they have to come to you as almost know one can outshoot the dwarfs. If your attacking, you take atvantage of the relentless rule.
I've found that due to there low movement trying to hid them behind cover sometimes means you miss out on an important charge

The most negative way that I find that some dwarf players adopt to negate enemy movement advantages is to just simply let the enemy come to them. Set up a fortress formation and let the enemy advance whilst pounding them with whatever firepower is available. This is successful, not due to any outstanding tacticians but due to the dwarves own artillery prowess. However this makes for a tedious game and if an opponent knows what he is doing with his army then this tactic will not be successful for long. If you are being outmanouvred and being picked of in combat then just dont let it happen. The 3 inches of movement combined with the relentless rule mean that although you may not get the charge you should still be in a position to flank attack with a different supporting unit in the next turn. The unit should always be able to hold against the charge due to tgh4, arm etc. You know that realistically you arent going to go zooming across the board like roadrunner on speed but it doesnt mean that you sit there and take what hes got without a few tricks up your sleeve.
Focus on your objectives of the scenario and if you give each unit their own little objectives after deployment just stick to them.