As mentioned above, headers, exhaust and intake are the best places to start. There isn't a whole lot you can do to a 4A-F engine in terms of performance, but there are a few things that will help. Upgrading the carburettor will definitely help. A Weber carb would work well.
Also consider the simple things, such as changing the leads, plugs, distributor cap, oil etc. If these haven't been changed in a while then that will definitely hinder performance. You can slightly advance the timing to get a bit more power too.
Although at the end of the day, to get some serious power under the hood, you'll have to do an engine swap. A 4A-GE 16v or 20v would fit the bill nicely, but if you're on a budget this might not be your best option.
Have a read of these threads for more ideas:
http://www.toyotanation.com/forum/t338839.html
http://www.toyotanation.com/forum/t29170.html