There are two ways to gain a lot of extra XP with minimal effort: Hacking and Non-Lethal Actions

Hacking: when every you get the chance, and if you have the appropriate level of hacking skill, hack everything you can. These simple tasks will earn you easy XP, and can even unlock extra goodies along the way (an access code, extra credits)

Non-Lethal Actions: Non-lethal takedowns/kills earn you more XP than you would with lethal ones. Also, if you have the Reflex Booster augmentation (which allows you to take down two enemies at once) you can earn an easy 125 XP from a NL takedown than from a lethal.