When hacking, save the game and hack the optional nodes-- but do not complete the hack. After hacking the nodes, load the previous saved game. Repeat the process. When you finally complete the hack, you will get all the nodes (from the previous times you loaded) along with the nodes you get normally. Use this on the computer in Jensen's office for 500 experience points every time you load for easy Praxis points.
Hack everything you can if you have the required Hacking skill. This will result in easy experience points as well as possible access codes and credits.
Non-lethal takedowns and kills earn more experience points than the lethal variety. Note: If you have the Reflex Booster Augmentation, you can will earn 125 experience points from a non-lethal takedown.
In Hengsha, China every store has a back door. Open it and walk inside for a Traveler bonus of 100 experience points.