Someone just emailed me a similar question, so I thought I'd post my advice here, too...
I think what makes a really good psychology essay is something you can debate, that a lot of studies have been done on. Bilingualism is a common topic for extended essay I think because it has a lot of interest among IB students (many are bilingual), a lot of work has been done on bilingual language development, and there's a lot of debate as to whether being raised bilingually is helpful or not (but the general agreement today is that it's a good thing, but it was very controversial 20-30 years ago). It's not a bad topic to pick if it appeals to you.
My topic was titled something like "simultaneous balanced bilingualism: does being raised with two languages help or hinder language development?"
The key is to pick a topic that is very specific (otherwise you can't adress the question well), with two arguments (so you can debate), and something not too obscure (so you can find information on it). If you think a topic might be good, type it into scholar.google.com and see what comes up.. if you're already getting some useful information it could be a good topic (as long as it's specific enough!).
On the other hand, you could pick something to do with adolesence or social psychology, as long as your topic is specific and has two opposing views. I'm weary of topics like "adolence" because they're not specific enough and "first impressions" because that's not really a topic much credible research has been done on. If you're interested in abnormal psychology for example, you could write about the high incidence of diagnostic rates for depression among adolecents and whether the high diagnostic rate means either psychologists are overdiagnosing teenage angst as depression or it could mean that adolencents are getting the help that they need.
Is this helping? If you want more ideas, let me know! I'm not an expert, but that's the sense I get from writing my own extended essay and seeing my classmates write theirs...