Whenever i'm listening to a DVD, the sound always seems whacked, the music or sound effects are always louder than the rest. What are the best volume settings for a nice surround effect where I won't have to fiddle with the volume every 10 seconds? Mine are at 0 front left, 0 front right, +12 center, -2 sub, +12 left rear, +12 right rear, but even with this, everything is louder than the voices.

The best way to balance the audio for all the channels is to get an SPL meter from radio shack $29. Your receiver probably has a white noise generator that will send a signal to each speaker and then you would adjust the volume of each speaker to the same volume. I like to have my center channel a couple of notches louder because the dialogue comes from that speaker. From what it looks like in your setup you have your rear speakers too loud.

Every speaker should be the same loudness when perceived from the listening position.

I'm sure your receiver has a pink test tone. When you press the test tone, a pink noise will emit from each speaker. By ear, adjust each speaker's volume until they are all of the same loudness from where you usually sit.
If you want to be more precise, then go out and get an SPL meter from Radioshack to measure the loudness of the pink noise test tones.

The reason why dialogue is low is because unfortunately, movie soundtracks are extremely dynamic in range. Dialogue is generally recorded low so that explosions will really blow you away like in the theaters. You're compensation this by jacking up the center to +12dbs but this really throws your system off balance. You probably aren't hearing left to right pans as you should since your center is so loud.
There are some things you can do to reduce the dynamic range. Your DVD player (or your receiver might also) will have an option called "Dynamic Range Control". Check your manuals to see how to turn this ON or to MAX. This will reduce the dynamic range and in essence keep the volume at a constant level while raising the whispers and reducing the booms.

I've found this to not really work that well though.
You could get a compressor from dbx or SIMA makes an inexpensive one also. I have the SIMA product and its called a volume stabilizer. It works only for analog connections and what it does it raise the quiet sounds and lowers the loud sounds so that everything is more level.
Check out: http://www.simacorp.com

Hope that helps.