First of all, you should inquire with your city's arts council on whether they allow bagpipes played in public. Many cities don't permit loud instruments, and most cities designate certain places for performances. Most importantly, some cities require permits. Before you go out in public, you should check out what your town requires you to do. You don't want to accidentally break a law and get harrassed by the cops (even though most cops love pipes).
You should limit your load to what you can easily carry in one trip. Your instrument bag, a money-jug, and supplies should easily fit on a small luggage-cart, or on your person. You don't want to make multiple trips to your car, if you're parked far from your spot. Also, if you use public transporation, it is doubly important that you pack lightly.
You should include in your load, some snacks and a jug of water. Piping dehydrates you, and you will get very thirsty while playing, especially if it's summertime. Among your gear, you may be tempted to carry a sign explaining your instrument, or a music stand. Don't bother, unless your sign can stand up to a good wind. I used to take a music stand with me, but it would always blow over. I switched to using a sign that folds out and can be weighted down with a rock in my instrument case.
You should also have business cards. People will always ask for more info as they walk by, or if they wait around for you to stop.
Well, you know what you've gotten yourself into, since your instruemnt of choice is loud. You'll need to keep track of where other musicians are around you. If you're the only one, great, but if other people are playing guitar, string quartets, portable keyboards, or other instruments, you don't want to piss them off by drowning them out. If there are rules about scheduling in your city, you'd be best to keep tabs on the other musicians to work out a schedule with them. The best thing to do is give the other musicians a good warning. You may want to walk around and ask them if you're too loud. The more you go out of your way to talk to other performers, the better you will get along with them. Musicians appreciate being given a warning, and will be less likely to be angry if you are.
If you have the ability to play every day, don't play in the same place day after day. People nearby will get very annoyed very quickly. Choose several locations, and rotate between them. This way, people will not see you as a constant annoyance, just an infrequent one.
We all have to deal with drunks, homeless people, crazy people, mischeivous kids and rude old farts, but we have to know how to deal with them so that they do not bother you or your audience. First of all, select a spot where there are plenty of other people regularly passing through. You don't want to be isolated, because if a violent person assaults you, you'll be left on your own. Sticking to peopled areas with lots of foot traffic not only gets you more money, but offers some potential deterrant to derelicts and whinos who may annoy you. Your audiences may even help shoo them away.
Secondly, If a drunk or insane person tries to talk to you, ignore them, or keep playing. They want to be your friend, and trust me -- you don't want to get acquainted with them. They will either try to take money from you, or they will keep talking to you and distract you. If they persist, you will have to ask them to please stop bugging you, but ignoring them tends to work. Threatening to call the cops or park rangers seems to work well, as many cities have laws against public drunkenness.
I was violently attacked by an insane old fart once, who objected to my costume of all things. Apparently, my medieval robe looked too much like a monk's outfit, and he got some idea that I was impersonating a monk to get religious people to give me money, or some foolish idea like that. He started ranting and raving, and kicked my money-jug away. I told him I'd get the police, and he ran away. The guy must have been in his 80's, too. This can happen, and the best defense is still a populated area.
Talking to people is inevitable, and the only problem is talking for too long. People will have questions about your instrument, if you're in a band, if you've heard of this group or that group, and if you'll play funerals. People always ask me to play amazing grace. The more you talk, the less moeny you'll make, although some people give you a five-dollar bill if you talk with them. If you have a time limit to play in, talking will give you less time.
One way to stir up interest in passers-by is to play an unexpected song. Nobody expects to hear bagpipes playing TV theme songs or 80's pop tunes. I highly recommend learning a few tunes, like Gilligan's Island, The Brady Bunch, and other familiar tunes. People will laugh, and they will appreciate it. You'd be surprised to find out how many times an unexpected tune on bagpipes will draw people near.
Remember, it's not just about the money, you're getting valuable practice time. If you're learning new tunes, this will be your way to practice them or test out variations. If you suck, then you may want to wait before busking, but if you are reasonably competent, you should be able to start pulling in $20-$30 an hour playing pipes (well, depending on how many people there are, and whether they actually like bagpipes). Staying safe, prepared, and hydrated is important. Hopefully, this advice had given you enough ideas to avoid common mistakes.