Thanks for all the advice!
Especially regarding the rule of thirds for the portraits, and using Kelvin-mode to remove the yellowish look of the indoor photos. Regarding the flash to remove shadows... I actually bought the 5D Mk II in order to NOT use a flash; to achieve a more "natural" looking light. But I probably have to master the flash sooner or later, if I'm taking photos in a club or similar. I have the Speedlite 430 EX II, but I have actually not taken a single photo firing the flash. I only use it for the AF-grid in dark environments, which works really well
Any quick tips on learning how to use the flash? For the purpose of mainly filling, like in daylight situations when parts of the face might be in the shadow? But as I say, I really want to keep the "warmth" of indoor lighting.
I understand that it's a trade secret in itself, it takes time to master; that's why great photographers can charge what they do.