You should be able to profit very well from raiding. Here's a few approaches that can work well:
1. Go around "skimming the cream", i.e. just move into counties and take the loot that is not protected by fort level. Check the counties first - I look for clusters of counties with at least 25g in each, though preferably closer to 50g in each. This is the only way to reliably raid strong realms, like Germany, France, Spain, etc.
2. Raid theocracies and republics. Since the capital holding is a city or temple, it's easy to siege down and loot. Likely targets are Rome, Amalfi and Venice, all of which I've raided for good profit (oh and they're all in the same area too). Also remember that you can sail up and down major rivers as a Germanic raider; I've sometimes raided lone theocracies far inland in Europe.
3. Raid for ransom. Some players say it's not worthwhile to raid tribal counties but I'm not sure I agree, given how much you can get from ransom. Tribes are easy to siege down and if they're the capital of a chief or high chief, then you're likely to capture most of the women and children in his court. For good returns I'd say look for a county that has a temple too, and a ruler with a large family (heirs can be worth 46g each). Oh and make sure the guy has money to cough up
Finally, on point #3, I've also used it to get a woman I wanted as a concubine (a 16 year old genius of course). They wouldn't accept my courteous invitations or requests for conubinage, so I decided to send my ships over to get this lady the Viking way

Worked like a charm, though she wasn't very positive to me at first.
Oh and make sure your ruler gets the Viking trait if you're Germanic! You need to have him heading the raiding army and then complete the siege of a holding. You don't always get it right away, but it's fairly easy to get.