A magnet can hold stuff for you while you are tacking things in place. Here are some 'booger' prevention tips I've learned from practicing:
First, if you are using a gas mig welder make sure your gas regulator is adjusted for the proper flow.
Second, having too much wire stick out will make the boogers as well cause the gas is not able to sheild the weld from too far away. Stick out is the length of wire that comes out of the end of the welder....so try to keep this to half an inch to an inch max. An easy way to do this is to use a pair of side cutters to cut the wire off to the proper length before you make your next weld.
Allways positon yourself so you can see the weld pool or you will just burn through and make mistakes. You can hold the gun on a slight angle and push or pull the gun to help you see what you are doing.
Just use a series of tacks to weld sheet metal. Space your tacks all over your work area to minimize heat warpage.This is called stitch welding. If you want to lay a bead of weld dont lay a bead longer than inch between tacks or you will have unreapirable warpage.
Miller and Hobart both have excellent websites with tons of info to help you trouble shoot your welds.
Hope this helps and good luck!