If you plan on buying in April of next year, I would start looking in a couple months just to get a sense of value. Prices could go up or they could go down. Since many homes now are short sales it's possible that you should start having an agent show you homes about 6 months before you move date.
The Carrabba Group
Keller Williams Sunset
Start looking in the fall of 2010--like this September or October. That'll be more for you to get an idea of what sort of properties are out there, and at what price ranges. Different areas have different buying cycles, so check with a local Realtor. However, in many areas, activity dies down in late fall and then comes back in early spring. If that's the case around Downey, I'd suggest looking before early spring. And because a lot of people take their homes off the market in the Thanksgiving/Christmas/New Years range, you'll probably get a better sense (and have more properties to look at) if you start looking in September or October.
It's true that market conditions can change in the 6 or so months between the time you start looking and the time you really buy. But at least you'll have a much better idea of prices and availability if you start early, rather than waiting until, say, next March.
One other comment: If you plan on selling in April 2011, you should start getting your home ready several months in advance. Painting, repairs, new plantings, and so on. That always takes longer than expected, and you want to be able to hit the market with your home in pristine, attractive condition.
Again, a local Realtor can advise you on market conditions, when the buying season begins, and so on. But, generally speaking, it's better to get an idea early on about what's out there. And it's certainly better to get your own home in really good condition so that it's ready when you put it on the market.
Hope that helps.
Coldwell Banker Allstars
13915 San Antonio Dr.
Norwalk, CA 90650