Most needlepointers like to use the lasso loop method of attaching beads. Essentially you use a doubled length of thread, come up through the bead and go down again, then come up in the original hole, let each thread lie on either side of the bead, and go down again in the original hole. The thread lies on either side of the bead, trapping it in place. You can see a good demonstration of this technique in a Needlepoint Now magazine video.
This is not the only way to attach beads, however. This author of the Beadpoint book uses continental stitch in vertical columns. Please note that she is covering her canvas entirely in beads. If I were just adding a bead here and there, I'd double my thread and lasso each bead for security's sake.
When I covered my SharonG bra and tap pants set entirely in clear beads, I worked from the bottom left corner up in basketweave, using the lasso technique to hold each bead in place. While working this piece I tried a lot of ways to attach the beads and this bottom-up basketweave technique (with doubled thread so I could lasso each bead) held the beads in the correct place better. It takes a long time, though! I wouldn't recommend it unless you have a huge area to completely cover with beads (and you are VERY patient).
You can use an X stitch to attach a bead. Robin King used a cross stitch through her beads to make a beaded cat face. This allows each bead to line up squarely with its fellows even though this is not a solid mass of beads.
I've just seen an example of a solidly beaded area done in diagonal rows like basketweave using cross stitches in the way Robin used them, but that changes the direction of the beads each row. One row has upright X stitches holding the beads, the next has the X on its side. This technique is Don Lynch's invention and is used on the candy corn block on his witch's hat. It makes the area of beads look woven. Don is with Associated Talents if you are interested in this new design. Very creative, Don!
Vicky DeAngelis wrote a nice article about solidly beading an areas on 18 count canvas on her blog. She has tips about needles and the size of beads she likes to use for an area that is all beaded.
Robin King likes to use brick stitch when she has an area that needs to be solidly beaded. Note the difference between the look of her beaded area and Vicky's picture. It's the difference between a straight and a tent stitch when it comes to attaching a bead.
Sundance has a nice booklet on beading needlepoint canvas. This is the only such book I know about. Here's the review I wrote of it in 2009. The techniques above are more about using straight or tent stitches to bead but Sundance has taken this a step further and subsstitutes beads for patterned stitches and has their favorites diagrammed in the back of the booklet.
I'd forgotten about their Bead Explosion kit last time so here's a glimpse of it. The difference between this and Sundance's little beaded heart kits is the variety of beads and stitches used. If you are interested in branching out into more decorative effects and seeing a variety of bead styles but want some help, it's worth getting this kit.
There are a lot of ways to bead, depending on what you are trying to do. I hope you experiment a bit and discover what works for you on your current piece.
Now that you are sick of beads, here's a link to show how much beads add to a design.
Written by Jane/Chilly Hollow Blogging at http://chillyhollownp.blogspot.com and at http://chstitchguides.blogspot.com