When teaching my kids to slice bread, I start with them on a stool so they can look down directly over the loaf. That way you can see both sides of the bread and guide the knife to a straight down cut. Like others have said, make sure not to push, let the serrations do the work. If you make a mistake, don't try and match the crooked cut, just cut one thicker slice that is straight down to start off with a good side again.
An electric knife does work well if you have one, but don't buy one if you don't. Same rules applies, get yourself directly over the bread in order to make a nice straight cut.