ok here`s the problem,while im playing in standing position with my strap (nylon) i cant find a comfortable position,i mean i play like 90 degree while holding the guitar,but the neck keeps falling down,so i have to hold it with my hand and it really bothers to play with the hand while still holding all the neck weight,is there any straps that can hold the guitar in a position im comfortable and stuff? sorry for my english and stuff
The problem sounds like your basses balance more than anything a strap can do, what model is it. there may be straps designed for head heavy basses but I don't know any offhand.
That Mad English Child 0.o
Gear: -
Fender Stratocaster
Les Paul custom
Washburn Force 4
Rickenbacker 4003
Self-hand made Electric guitar
Self-hand made Bass
and a rubbish acoustic =)
You may want to look at getting a wider strap and maybe switching to a leather one. Yes, they cost more, but so does a chiropractor or physical therapy when you strain your shoulders or back.

I've never had luck with wider nylon straps with my heavy Accubass. Wide leather straps are just so much more comfortable and better for my back and posture overall.
An alternative solution that no-one's mentioned so far is moving your strap button. If you're having a real problem with neck-dive (the headstock effectively needing holding up) and not a problem that can be solved with a wider strap or adjusting the height then a lot of people have solved it by moving the strap button or strap lock (the metal thing that the strap attatches to on the bass). By doing this you can even out the balance of even the heaviest and most headstock-heavy of basses - problem solved.
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You might want to put two strap buttons on the rear bout of your bass: one each about three or four inches from the centerline. This is what Status does to balance their basses, and since Mark King is one of their biggest endorsers and he plays his bass at a 90-degree angle the way you do, it might very well be the solution you are looking for.