Here's what I do. About once a year, I look ahead: How many 1 year, 2 year, 3 year, etc. b-day parties will I have in the next year? And I estimate based on the kids' friends.

Then, I buy...

1st birthday presents: Megablocks or pajamas and a goodnight story
2nd birthday presents: swim towels (from Disney when they have sales )with pool toys
3rd birthday presents: bath toys
4th birthday presents: Word Chase board game (beyondlearning.com)

etc.

I get it all whenever I see it discounted and buy buy buy ahead.