What you don't want to do is try to glue end grain to end grain -- won't work well and no mechanical reinforcement.
As to dowels or biscuits, that depends on the use. If the long board is simply decoration without any serious bending, twisting, pulling etc then they can work okay.
A lap joint or a scarf joint lets you get some long grain to long grain bonding, which is much stronger. Again, it will depend upon the kind of pressures that might be put on the board. Mechanical reinforcement to a lap joint could include a peg, but that is more visible even than the line where you join the two together.
There are many joints you can use -- however in general it is better to just use a board of the desired length. the joint itself is a relatively weak point that you are introducing when you join two boards together.
You can join boards end-to-end but I wouldn't want to rely on it structurally except possibly in a vertical application. Even then it would be a case by case call. Use biscuits or dowels to join them.
Now it's a whole different story if you're boards are a little longer than 1/2 of the final length. From your example, say 2'-6". Then you take the last 6" of each board and make a half-lap joint that will be much stronger than end-to-end as you describe. Or you could opt for a tongue & groove joint. The structural integrity depends on the amount of overlap, and precision of the joinery.
You might browse through your local bookstore. There should be several books on Joinery that will describe this and many other techniques.
Like Carl said, a scarf joint works well to do what you want.
I repair vintage aircraft. Wing and other components are built from clear spruce, tacks, recorcinol glue and doped fabric. The FAA specifies a scarf joint for repairing spar (long, usually horizontal frame members) damage. The joint must be 12 times as long as the spar is thick. So a 1/2" spar repair joint is 6 inches long. They also require gussets over the joint ends, but you don't need to do that.
Just yesterday, we used clamps after glue simply to take the bow out of the wood we were using, and it straightened everything out for us. Usually though, any time you lay up a project the clamps are...