The parks are busiest during the summer, around Easter and Thanksgiving, and again between Christmas and New Year. Crowds thin out from January to March and November to mid-December.
If you're visiting during peak periods, try to visit midweek instead of weekends and aim to arrive at least half an hour before the gates open, then head for your favorite rides first. Lines are usually shorter at lunchtime and during the parades.
If you don't mind riding alone or with a stranger, several attractions in both parks have “single lines” to fill any gaps. Usually these lines are much shorter.
Each park has baby care centers, boutiques ...