Asking a guy to a dance is always a tricky thing to do. Each situation and each guy is different from the next, but there are a few easy methods you can use that can help your chances of getting the answer you want.
Sing It To Him
An easy way to both make a guy laugh (in a good way) is to sing your question to him. If you're creative, write a song for him where you ask him to the dance. Don't be too sappy or pushy, just keep it lighthearted and funny. If writing songs isn't your thing, take the lyrics to a popular song (or for bonus points, his favorite song) and change them so that they ask him to the dance. For this method, you can either sing in person, if you are very brave, or simply leave him a voicemail.
Get Someone At An Event To Ask
This is a method for the brave, but a special way to ask out a boy is to get the headliner at an event to do it for you. Think rock concert or a stand-up comedian's show, or if you both have something special like a birthday party or wedding to go to, that could work as well. Approach the person headlining or whose event it is beforehand and ask if they would be willing to stop the event for a second to ask the boy you like out to the dance for you.
Write It In His Yard
A classic yet classy way to ask your desired guy to the dance is to write it in his yard, though of course you'll have to wait until he's either out of the house or asleep to do it. When using this method, be creative in what you write and the way you write it. For the message, keep it simple, and try to reference something that relates to your relationship. For the how, you can again reference a memory you have together, or do something that looks interesting, such as using Christmas lights or something reflective that will catch the sun.
Make A Scavenger Hunt
For a guy who is already your friend, set up a scavenger hunt with the question at the end. You should only try this with guys you are already friends with, as otherwise it may be difficult to get him to actually do the hunt. When making the hunt, think about your relationship with him and have him go to different places that you have memories together, and have him find clues. Don't make it very long, or he may get distracted, and keep the places and activities to find clues fun and fairly simple. When he gets to the final location, ask him out! You can either do this in person, with a message at the location, or even leave a phone there for him to call you.