My bicycle seat never stays put?

i got a new bike but the problem is no matter how hard i tighten the bolt the seat keeps moving left to right and makes it kinda scary to ride a bike. please help me im trying to do my part for global warming

the bike is a BOSS Stealth 18" 21 speed with full suspension and magnetic brake discs

Can you provide the brand and model of the bike? There are some specific things that you can do, but we need to know the brand and model.

In the meantime, make sure that the bolt holding the saddle onto the seat post (under the saddle) is tight.

EDIT: OK, here is a "fix" if tightening the saddle onto the seat post didn't work.

1) Remove the seat post from the bike and remove the bolt from the frame. If equipped, remove the binder clamp;
2) Using a screwdriver, slightly spread the frame at the slot... not too much!
3) Get an aluminum soda or beer can and cut a piece from it about 2 or 3" wide and long enough to wrap around the seat post once 1/8" or so short of overlapping. You can use scissors to cut the stuff;
4) After reinstalling the binder clamp (if equipped) wrap the piece of metal- a shim- around the seat post and reinsert. Make sure that a small amount of the shim extends up out of the frame. Carefully bend this edge over.
5) Reinstall the binder bolt and tighten carefully.

Do not start altering the frame or packing out the seat post, you could invalidate your warranty!
This is really the responsibility of the retailer to sort out, but I guess you have bought this mail-order and are on your own
First, check if its the saddle moving on the stem and not the stem in the frame, if so, look at the clamp immediatly below the saddle, make sure this is fully engaged on the stem and tight, sometime the seat post is not fully inserted into the clamp on the saddle.
If the seat post is moving in the frame, do you have a quick release bolt, these can be difficult to get tight enough, change this for a bolt if you can.
There is a very remote possibility you have been supplied with the wrong size seat post, a shop could sort this out for you.
If you cannot solve this easily yourself, talk to whoever you had the bike off, if its a shop, they will sort it, if mail-order they could tell you to go to a shop and they will pay.
EDIT, I disagree completely with those saying open up the frame and fit shims, you could break off the bolt lugs and destroy the frame, absolutly the last resort by someone who knows what he is doing, talk to your supplier first.

Do not ride a bike with a moving seat, it could be dangerous.
Most likely there is a problem with the bolt and nut. They are either wrongly placed or not the right size for your bike. It seems you have tighten them all they need to, so that is not the problem.
The easiest solution is to take it to where you bought it from and have them have a look at it. And you needed change the bolt, or nut. It should be cost free and it is something quite cheap for the shop anyway.
With more details as far as model and made you could get other alternatives form the people like miragou and the rest.

try taking it back to the store you bought it. Tell them the problem and hopefully the will give you a refund, or fix it. If this doesnt work, write a letter to the headquarters of the store complaining. they usually do something then. :) good luck

It's probably the clamp. A lot of people overlook this; it might not be the seatpost - it could be the clamp that attaches the saddle to the top of the seatpost.

FAO - Mirageguitarworks. Thats a great suggestion, i'll do that myself!

gosh , these questions get better and better