Here is a very simple rule of thumb for going up in weight:
Decide how many reps and sets you are doing. Let's say you are doing 3 sets of 12. If you can do the last rep in the last set without struggling, [i]slightly[/i] increase the weight until you are struggling to get the last few reps in that last set. And always have a spotter. If you don't have a spotter, stick with what you know you can lift.
Easy stuff.