Ive been playing bass for about half a year, and I'm really good i think, but i'm completely self taught, so i have no idea if my fretting technique is correct. Sometimes when i play through certain songs (harder ones) my fretting hand will start to hurt right between my thumb and index finger. I was wondering if this is because of technique or if i simply need to get stronger. If it helps my technique is generally to have my tumb on the exact bottom of the neck underneath my index/middle fingers.

Well, I got you "beat" by about a year -- been playing a year and a half. Piano's my first love. But I've I had the same problem until recently.

Now I make a concerted effort to pivot my whole fretting hand more, along the direction of my index finger toward my body/away from it. (Am I making sense?) In other words, I'll pivot my arm/wrist so my index finger gets closer to the low E, and my thumb can move closer to A. I don't hold my hand that way, understand -- but as I play I exaggerate this pivotal motion and it keeps a cramp from developing.

Basically, I throw more arm and wrist into playing. Stretching before play helps a lot, too. Don't undervalue that.

What do you experienced bassists do?

Ya know, Gretch, I'm really glad you asked this question. For all I know, my "answer" might be the path to a really bad habit! I hate undoing those things. Help, anyone?

Try using a little less pressure. Have someone set up your bass and lower the action. The action is how high the strings are off the fretboard. Get the action as low as you can before the strings start causing fretbuzz. The less pressure you have to exert on the strings will make the pain less.
Also, try relaxing your thumb a little. I lay my whole thumb across the back of the neck almost wrapping it around to the E-string.
Also try making your reaches and movements more fluid and try to use all 4 strings instead of flying all over the board. Learn where the notes are and use that to your advantage. Remember that there are more than 2 or 3 places on the fret board to play any note.
I've been playing for 14 years and my hands still get tired. Of course, that's after I've been playing for 3 or 4 hours. The soreness is something you'll have to work through, even with doing any changes, just the same with the blisters. Good luck and welcome to the world of bass. It's a lot of fun.

Well, Gretch, I have several suggestions for you. The first is to be careful, because if your hands hurt every time you play, you're doing something wrong, and you're going to do some damage that will come back to haunt you someday. The second is to evaluate whether you're holding the neck too tightly. Your hand should play the neck, not strangle it. Try this exercise -

Play a scale up and down very slowly - half notes at 60 beats per minute, or one note every two seconds - while your thumb is NOT TOUCHING the neck. Just let the thumb dangle in the air, and try to let your fingers do all the work. Now, you're not going to ever PLAY like this, but doing this exercise over a period of time will help you lighten your touch and loosen your grip on the neck.

This would also be a good point in your career to evaluate your technique, and the best way to do that is to record yourself. Do you hear ringing open strings, an excess of string noise, smooth transitions between some notes and awkward jumps between others, uneveness of tone or volume? These are common technical problems that every serious bassist has to face sooner or later. Tape yourself playing something, and listen carefully to the playback. It might indicate technical difficulties that are causing you to struggle, things you'll need to work to improve. Hey, it's better for you to hear it at home and improve it now than have your audience hear it later.

I'd also like you to consider whether you might be stretching your hands too much. Maybe you'd do better to shift a little more if your hand is aching. Are you playing with your fingertips? If you're not, you should strive to.

Try to keep your technique clean and your hands relaxed at all times. You mentioned that this problem occures on the "harder songs." It's natural to tense up during more difficult passages. Try practicing in a slow, relaxed way and let your technique develop cleanly. You can't FORCE yourself to get better, but you CAN discipline yourself to keep your technique CLEAN while you're practicing, and this will pay big dividends in the future, even if it feels frustrating at first.

Last but not least, ask a reputable teacher for a one time consultation to discuss any problem areas that you can think of. It will be time and money well spent.

Good luck!