You can use the "Easy gold" glitch to basically get unlimited experience. Once 
you save the life of the Arl of Redcliffe, a soldier of his will appear in your 
party's camp. There will be a box for donating to the army. Enter the screen, 
and select the first option to donate 30 gold. You can do this as many times as 
desired to get a good amount of experience from each donation. If you just keep 
pressing X then Square repeatedly, you will keep quickly donating large sums of 
gold, skipping through the dialog screens, and getting a lot of experience 
quickly. If you start getting low on gold, perform the "Easy gold" glitch again 
with the merchant in your camp. Note: Be careful, as enemies will also level as 
you do. If you get too high of a level and do not have the correct equipment, 
you can be quickly defeated.

In Ostargar, do the "Tainted Blood" mission that is unlocked immediately before 
you become a Grey Warden. Collect the three Vials of Darkspawn Blood, but not 
the Grey Warden Treaties. Return to camp, talk to Duncan, and report "We have 
the blood, but not the scrolls." You will exit out of the conversation, but 
still get the experience for the mission. You can repeat this as many times as 
desired. It does not take long to reach the level 25 cap if you skip the 
conversations to speed up the process.

Note: This trick requires you have the Trap Making ability. In Lothering, go to 
the right after exiting Dane's Refuge (near the center of Lothering). There will 
be a quest giver by the name of Allison. She will ask you to make traps, and 
requires three traps to complete the quest. Once you complete the quest, you 
will get a moderate amount of experience and 50 silver. You can repeat this 
quest as many times as desired; however, the quest will become unavailable as 
you progress through the game. To keep repeating the quest, go back to Dane's 
Refuge, and buy Trap Triggers. He has an unlimited supply of them. As long as 
you have Trap Making, create Spring Traps. Spring Traps only cost approximately 
3 silver to create, and she requires three traps each time. Thus, it only costs 
you roughly 9 silver for the quest.

To get extra experience points early in the game, go to the city of Denerim as 
soon as you leave Lothering. Then, enter the "Wonders of Thedas" building, and 
purchase the Archivist's Belt. While it is equipped, you will get an experience 
point boost for everything in your Codex. Books will increase from 50 to 75 
experience points.