Meguiar's also makes a nice cleaner. The Gold Class Bug & Tar Remover. It is pretty mild, which is what I like about it. I'd guess Mother's might be similar. But then that means bugs and tar must be fairly fresh, and you need to give them 30-120 seconds or so to work on the bug guts. Remove what you can with the wash, then try something mild like that. Clay can also help. If mild ones don't cut it, time to step up to more solventy stuff that may temporarily dull the finish a bit. Stay away from the $.99 stuff at the parts stores. They generally don't remove anything but a dollar from your pocket.
As an FYI, you can use a product like ProWax's Bug Guard prior to such trips. I've just started using it, and bugs just wash right off. The Bug Guard forms a temporary barrier, and once you hose it down, it starts to break up taking the bugs with it. This also means the Bug Guard won't last through a rain, though. It's a short-term solution but is quite good when used within its capabilities.