Lamination doesn't necessarily have to cost an arm and a leg. In fact, at a frame or print shop, the cost you pay is mostly for the convenience of having the lamination done for your. Here are two easy ways to laminate for less at home.
First is the ready-made way. Go to your local office supply center and find the laminating envelopes. Choose envelopes larger than your piece so it is sealed on all four sides. It can be trimmed down later. Depending on the lamination system you choose, you may need to purchase a lamination machine. Cold press lamination will always be less expensive than heated lamination and probably cause less damage to your piece.
When you get home, simple center your piece between the lamination sheets, run through the machine (if necessary), and trim off the excess. Quick and cheap home lamination.
The second method is a bit more crafty. The only supplies are your piece, clear contact paper, and a good pair of scissors.
Spread out the roll of contact paper on a flat surface. Center your piece on the contact paper. Cut around your piece, leaving a good sized margin that can be trimmed down later. Repeat the process to get a second piece.
Starting in one corner of your piece, remove the backing paper from one sheet of contact paper and press down, leaving a margin on the outside of the poster. Slowly work your way to the opposite corner, lifting the backing paper off and pressing down to prevent bubbles from forming. Flip the piece over and repeat the exact same process with the other sheet of contact paper. Trim off the excess and you have a very inexpensive laminating alternative.
A few words on lamination safety: Do not laminate anything that is irreplaceable. Anything can happen during the lamination process. The adhesive on the lamination may go through your piece. The lamination machine may jam, destroying your piece. Furthermore, lamination permanently reduces the value of anything you run through it. While you may want to preserve a signed poster, it is probably best to spend the extra money and have anything of significant value custom framed.
Furthermore, lamination is not the same thing as waterproof or outdoor safe. Lamination provides a plastic coating on the outside of a paper, though it can still easily get wet or be ripped apart outside. For items you really need set for outdoor or waterproof uses, safe yourself the trouble and go to a print shop. They'll know what to do.