Increasing ROF Without a LiPo

 If you want to effectively boost your rate-of-fire, but don't really want to get into LiPo's, here are a few alternative options:

1. Buy a good 9.6v battery: Most start at $20-$25 for 1600 mAh. If you can buy it from an RC retailer or directly from a battery dealer, you may be able to get one cheaper.
2. Rewire to Dean connectors and eliminate as many connection points as possible. The Tamiya connectors that come stock in virtually every AEG are notoriously inefficient. The cheapest place to buy Dean's is probably Ebay, where they run for about $1 per set.
3. Replacing the stock wiring with a better quality/thicker gauge wire allows a greater flow of current from the battery to the motor. Depending on the quality of the stock wiring, this may or may not make a noticeable difference. If you do rewire, leaving the fuse out will definitely help boost ROF; however, you do have to be cautious. If your gun stops shooting for any reason, cease firing immediately or you may fry your motor and possibly your battery. If you feel the motor or battery getting hot, stop shooting and give everything a few minutes to cool down.
4. Make sure your gearbox is shimmed correctly. An incorrectly shimmed gearbox can cause added friction between the gears. This puts additional stress on the motor and reduces battery efficiency.
5. Get a high torque motor. When paired with a good battery, this upgrade drastically improves ROF. High torques are generally a better option than high speed motors because they improve trigger response and you'll probably need one anyway if you ever want to upgrade the spring in your gun.
6. If you really aren't concerned about FPS you can also just replace the stock spring with a weaker one and that will help improve ROF.

A note on "High Speed" parts:
Most "high speed" parts (ball bearing bushings, motors, gears etc.) on the Airsoft market are geared towards low FPS set-ups and therefore tend to be less durable. If you are aiming for 350 FPS and up, stick with stock or high torque parts.