One thing you can do is choose a different color. Violets are blue. In other words, if you write a line but then can't find a word to complete your rhyme, try changing the last word to something that's easier to rhyme. Another way to solve this problem is to completely change the line to something else. Here are several poems where I changed the second line to something completely different to make it easier to rhyme the last line.
Roses are red.
Roses are red. Carnations are pink. They frequently smell good but sometimes they stink. So you see, if you can't think of a good rhyme, don't let that stop you. Instead of giving up because you can't find a rhyme, just change the word you are trying to rhyme, or change the entire line until you come up with something that is easier to rhyme.