I have new flat bars and grips. What's the easiest way to slide the grips onto the bars?
If you have access to a compressor, compressed air works well – use the hole in the end of the grip to blow in air while covering the other end of the bars with your hand (or something). The air escaping around the grip opens it up enough that it is easy to slide on.
Rubbing alcohol can be used as a lubricant. It will evaporate leaving the grips snug on the bars. Be wary of a lube that won't evaporate as the grips may tend to spin. Some people glue grips on, but that can make them pretty hard to get off without cutting.