Lubing can easily cut your times by 5-25%. I lube my cube around once every few weeks (when I notice that the cube needs it), but your cube might be different from mine. Some types of lube are silicone (I use Prestone brand, but that probably doesn't matter), water-based, and WD-40 (this is generally regarded as bad because it can deteriorate the plastic, but you can use it to intentionally wear down your cube in order to make it looser).
2. Don't rush.
Going slowly can allow you to see what case will come up next, whatever method you may use. This improved my times by at least a second. You should, however, go quickly on the last step of your solution (if it is just alg, as opposed to an intuitive step).
3. Use good stickers or tiles.
Bad stickers peel off and can make it harder to recognize what color is what. I use stickers from cubesmith.com, but cubesmith tiles work well too.
4. Try to learn lots of algorithms for different situations.
For example, there are algorithms for every situation for the OLL. Thanks to Josiah Magnusson for this tip.
