I've never played a game with dwarfs yet, but I know how to play warhammer as I am a previous Chaos general.

I was just thinking... My main opponent plays HE, and as we all know - HE's are fast. He especially likes Silverhelms, and I fear that they will be hurting my flanks bigtime. How would I avoid getting my infantry flanked by all his cav? (I don't like the idea of lining up my army in the corner of the table, to get protection from the table edges).

Thanks!

Use slayers as they are unbreakable, thunderers as they can move and shoot and can react to a flank charge with the move ability or a war machine like an organ gun.
If its a big-ish battle put some breakers on the flank with the Master Rune of Groth One Eye to surprise him. You could just put a decent sized dwarf warrior unit on the flank as they should have a decent chance to tie up the silverhelm charge without running off.