As ever, mom gets everyone else ready before she sees to herself.
But now it's your turn! And the main rule of thumb - don't overshadow the bride and don't look dowdy yourself!
So in that vein here are a few possibilities for the big day:
First hope on over to Spiegel.com where they have a whole primer on wedding wear:
What to Wear to a Wedding
While aimed at general wedding guests, I thought just about any of these outfits would be lovely for the mother of the bride or groom as well. My two favs:

Sadly the above only comes in Misses and Petite sizes.
Total Outfit = $228.oo - $258.00Come one, come all for this one - Misses, Petite and Women's sizes welcome!
Over at Ann Taylor:

I'm not mad for the color on this outfit for a wedding, it would be better in a warm cream color or a pale silver or blue -- but I do like the idea of a tailored suit - very chic for the moms!

Once again, not wild about the color. But if you want to go the formal lenght route a very nice silhouette.
And, of course, Ms. Ann only does Misses sizes! :-p
But speaking of the formal route - over at Norstrom.com they are definitely ready to hook you up! They have a whole Wedding Shop covering the gamut of the wedding party with a section just for the Mother of the Bride or Groom
They offer a wide range of sizes, styles and prices so do go on over and check them out. I'll just throw out a couple of samples from their collection:


Yes I know - that brown tone again. But the slimmery fabric and the cut of this outfit makes this one a winner for me in this shade! But if you just can't see yourself in such a dark color at the wedding this outfit also comes in a lovely blue, and a spring type lilac.

Classy!
So no excuse for not looking fab while you're beaming with pride in the pictures! And congratulations on shepherding your darlings to this happy day!