Use wrist straps when you get to a weight that you can't hold anymore. Don't use the straps until you have to though, and make an effort to keep lifting heavier weights without straps. Over time, your grip should start to catch up with your back strength and you will be able to hold more weight. In addition to this, you can try simply holding heavy barbells for as long as you can, and doing farmer's walks to develop grip strength.
You probably should've done a search...this has come up numerous times. I'll save you the trouble this time.
I recommend doing as much as you can and then using wrist straps. You should also be doing some grip training (do a search for that) to bring your grip strength up to par. If you aren't already doing so, start using a hook grip (once again, use the search function).
You're just weak at this point and will need to build up strength over time. Even when I was deadlifting 225lbs, I wasn't using straps. I built myself up to that and then worked on getting stronger. I used straps when I was having grip issues in the 275-300 range. Then I used chalk for a while, then I went beyond that.
Keep the reps low and work on getting your strength up over time. With a mixed grip and chalk, you should be able to work up to a much heavier deadlift.
I think I really had three problems which I interpreted as weak grip. 1)My grip was actually weak and doing endurance type training helped because thats the kind of muscle you have in your forearms. 2)Low general physical preparedness. I think my grip would slip because I was generally tired so my body wanted to let go out the weight. Again the above program helped. 3)I could barely reach around my own fat belly.
Any high volume body builder type program would help, assuming you have problems 1 and 2. And I do think these are the problems based on the weights you are using and whatnot.
The other thing is that you need to build tendon strength. That's a slow process. Not many shortcuts except for partial work. I don't think tendon strength will be real factor until you're moving at least three plates anyway.
You can get chalk and straps at either elitefts or ironmind. Ironmind has grippers and books on grip strength, too.
To help increase your grip strength now, you can do dead hangs from a pullup bar, plate grips, grippers from an athletic store (when you can do 3x12 then they are a warm up for you), towel hangs, wrist rollers and don't forget ZEB's static bar holds.
But don't start using straps now. Just increase your grip strength.