Building off of Karl Maciag's last post about Frequency Coordination I'd like to take it a step further and get into some things you can do physically with a rig to make it perform better. Most of it is based of one simple part of antenna theory. If you have a piece of metal near an antenna it affects it's performance. There's a lot more to it, like if the piece of metal is resonant at the frequency you're dealing with, how many wavelengths away from the antenna is it, and so on, but that's the basics.
Some antenna designs use this to their advantage. If you look up at an old fashioned TV antenna on some body's roof you'll see all these horizontal spines. They're all cut to specific lengths and are set specific distances apart to take advantage of the effect and amplify the signal coming in one direction and reject it in another. Paddle antennas are a similar design, the elements are just flatter and laminated in a sheet of plastic. Hold it in the right light and you'll see what I mean.
A lot of the issues with wireless gear in sound systems come from the interactions between antennas and other pieces of metal. Table legs, the edge of a rack, eleven other antennas in the same rack and so on. If you're not getting good reception it's likely because you've created some sort of ad hoc antenna system that's rejecting signal in a direction that you don't want it to. Repositioning a rack of wireless receivers can do wonders. Sometimes just moving each antenna a fraction of an inch is enough. If they're all touching when you open up a rack, move em all around so they're free from each other and at all different angles.
This brings me to the second concept that's a little less important now that almost everything is UHF (ultra high frequency) and not VHF (very high frequency). The wavelengths are shorter and to put it simply those little antennas have an easier time finding them, even if they're being reflected in off a wall or something. In a lot of cases it won't really matter that much. A good receiver can make do with less than fifty pico volts of signal, but the orientation of an antenna can still make a difference if a signal is marginal.
If the transmitting antenna is vertical, the receiving antenna will work the best when it's vertical too. If the antenna is on an actor that's tumbling around on stage your best bet might be to set one antenna straight up and angle the other one (in a true diversity system with two antennas that is). If you're using an antenna combiner and paddles it's less of an issue because you've got a lot of gain in the system but with the little rubber duck antennas on single pieces of gear, a little tweaking can fix you right up.
Beyond that, bigger and more expensive systems are using circular polarization. That's where the antennas are actually spiral and the only thing that really matters is the direction of the spiral. In a well designed system you can have two devices almost on the same frequency and if they're polarized opposite from each other everything works out.
This is by no means an attempt at writing the book on antennas. My years as a ham radio operator have lead me through hundreds if not thousands of pages of reading on the subject and I'm just trying to tweeze out a few little details that might help. For further reading I'd suggest a simple internet search and start digging into all that same stuff I read. I'm sure you'll find that just like pro audio, there's a lot of voodoo mumbo jumbo out there along with all the hard science.