I've always sworn by Obagi's Professional C-Serum for taking a couple years off. There's something very potent about the benefits of vitamin C, and you pay for them too. (Plus, it kinda sucks if you don't use it all before the expiration date.) That's why I kind of loved this totally random, do-it-yourself, super easy recipe for a quick vitamin C boost.

From makeuptalk.com:

Homemade Skin Care Recipe: Vitamin C Peel
submitted by Mark Jones, Brighton, UK

Tip description / details:
I have been getting brilliant results from a DIY vitamin C peel. The vitamin C helps stimulate collagen production and the acidity is low enough for a mild peeling action.

Ingredients:
1/8 tsp vitamin c powder (ascorbic acid), water. You must use pure vitamin C powder, which has not been mixed with any flavors.
 
How to prepare:
Dissolve the vitamin C in water
 
How to use:
Apply with a cotton wool pad. Leave on for ten minutes before washing off any applying normal moisturizer.
 
Results (observed by submitter):
Firmer, tighter skin from collagen synthesis. Bright, clear complexion from the acidity being low enough to act as a very mild peel.

Costs involved:
Low