Think the best thing to do is to be consistent, in other words don’t spend 100 now and then spend 1000 next month, try find an average that you can afford each month, and then slowly increase over time (if you are making money).
As far as what you can do with a small budget, well, there is way to much you can do, but you are only going to be able to afford 1 or 2, so test each one out on a month to month basis.
One thing you should NOT do, is buy traffic from those sleazy traffic merchants, you know the ones, 100000 visitors for $10.