PA speakers and home theater speakers are anything but flat response. I suggest you do what those of us without high end studio monitors (or a well treated mixing room) do - mix to the best of your ability, then burn a disc, listen to it in every possible place you can - your car, home living room system, a boombox, a friend's house, etc. Make note of of what sounds good and most important - what sounds bad. Remix based on the notes you made, and do it all again.
Learn to use the speakers/listening environment you have
A 'reference disc' will also help - a CD form someone that you know well and think is mixed perfectly. What's it sound like in your mixing place? Go for the same overall sound and feel, to start with, at least.