A variety of increases

Make one increase on the knit side
In between the stitch you have just knit and the next stitch is a bar, pick up that stitch and from the back to the front with your left needle. Insert your right needle into the stitch from the front to the back (which will actually twist the stitch), knit the stitch, then slide from the left to right needle. This increase is practically invisible and slants to the right. To make this stitch slant to the left, insert your right needle into the back of the stitch, knit and slide it to the right needle.

Make one increase from the purl side
In between the stitch you have just purled and the next stitch is a bar, pick up that stitch and from the back to the front with your left needle. Insert your right needle into the stitch from the front to the back (which will actually twist the stitch), purl the stitch, then slide from the left to right needle. This increase is practically invisible and slants to the right.

Median increase
Knit the stitch to be increased, but do not drop the stitch, then knit the stitch from the row below and drop the top stitch from the needles. (Editors, Vogue KnittingPantheon Books, New York, 1989.

A final increase that is not pictured here is the yarn over, to create a yarn over, simply wrap the yarn around the right needle from the front to the back by going under the right needle. This is probably the first increase most knitters do, at least inadvertently, when they are switching between knit and purl stitches and fail to remember to move the yarn to the front or back when changing between stitches.
Placement of increases is also important. If you are increasing in a stockinette pattern (all rows knit on the right side and purled on the wrong side) placement is not as important. If, however you are increasing in a pattern you must plan where to place your increase so that you can keep your pattern intact. In that case you may want to increase before the pattern or at the edges of the piece.
Increases are a necessary part of knitting and can be visible or invisible, try some different ways to increase to see what works best for you.
Again, if the pictures above are not clear enough the Vogue Knitting -Ultimate Knitting guide has pictures and illustrations that are very clear and it is an excellent reference book to have on hand.

