My favorite way is to heat up a big mug full of water in the microwave for 60 seconds, and while the water is heating, cut your butter into small chunks and put the chunks on a plate. When the water is hot, dump it out and put the mug top side down over your butter. Give it 10 minutes, and your butter should be soft enough to work with! My friend Elise at Simply Recipes also has a few other methods for you.