Possibly your best bet is to actually keep track of the multitude of [ANN] threads over in the altcoins section.
There are also coin launch calendars - but they are often woefully incomplete, so you might have to look at more than one. Just google 'altcoin calendar' or 'altcoin launch' and you'll get plenty of hits.
You can also try following @crypto_blog as they have started to tweet about launches that they think might be interesting on there.
Be prepared to spend quite a bit of time if you really want to be serious about it. Otherwise, joining a pool that automatically selects coins might be of interest. Example:
http://cryptomining-blog.com/2760-coinking-a-new-multipool-to-try-out/