There is a time consuming but very easy glitch to help speed this along.
When on planet Therum for the first time (meaning this can be done with a pre-existing character on a second or third playthrough), activate the mining laser (the code is AXBY) and then save as soon as you receive experience.
Then, load the save you just made. You'll notice you gained more experience. Save again, as quickly as you can, and reload the new save. Repeat as many times as desired.
The experience you gain scales with level, so this is faster than just killing hordes of enemies.
Therum is the planet you rescue Liara T'Soni from, and is located in the Knossos system in the Artemis Tau cluster.