Uniden DMA scanner basics

When I got my first Uniden DMA scanner, a BC346XT, I was completely baffled by the way the memory is organized. My old scanners had banks of channels, say, 50 banks, with 50 available channels in each. If I programmed only ten channels into a bank, the other 40 channels couldn't be re-allocated in another bank.  Uniden aimed to remedy this with a more flexible memory arrangement.
On a Uniden scanner, channels are nested into groups, which are nested into systems. So, on a larger conventional system that I listen to often, such as the city of Boston, I'll have it programmed in as such:

System: Boston
Group 1: Police
Channel 1: BPD Channel 1
Channel 2: BPD Channel 2, etc...
Group 2: Fire
Channel 1: BFD Channel 1 , etc
Group 3: EMS
Channel 1: BEMS Channel 1, etc

Because there are only 100 System Quick Keys (more on that later), you'll want to keep your total number of systems under 100. You can program in many more, having systems without Quick Keys, or two systems can share a Quick Key, but I prefer for every system to have it's own Quick Key. So, for smaller systems that I don't listen to as frequently, I arrange them a little differently:

System: Midddlesex County #1
Group 1: Acton
Channel 1: Acton PD
Channel 2: Acton FD
Channel 3: Acton EMS
Group 2: Arlington
Channel 1: Arlington PD
Channel 2: Arlington FD
Channel 3: Arlington EMS
etc, etc

Each system can have 10 Group Quick Keys, so I prefer to have no more than 10 groups per system. Because of this, Middlesex County takes up 4 or so systems on my scanners. Fortunately, there's still plenty of room.
Once you map out how you are going to assign all of your systems, groups and channels, you can program them in. Hand programming is possible, but using the computer makes it exponentially faster. I recommend the program Freescan, as it's just as good as any other program, and it's free.
Systems and groups are turned on and off by their quick keys. Because of this, every system & group should be assigned it's own quick key. As an example, on my scanner, Boston has a system quick key of 9. Simply pressing 9 on the keypad turns Boston on and off. While scanning Boston, I can turn the different groups (police/fire/ems) on and off with their group quick keys, by pressing "Func" and then the group number. I strongly recommend not using the Lockout (L/O) feature to turn channels on and off, as it can be time consuming, and can also lead to the glitchy "nothing to scan" error. The only channels I have permanently locked out are the ones that I definitely never want to listen to, for instance, BPD Channel 8, the listings and warrants channel. Pressing L/O once while on a channel you don't want to hear is acceptable, as it will only lock it out for this session, and it will be unlocked when you turn the scanner off and back on again.
Trunking systems are similar, although they are broken down into "Sites", then "Groups", then "Channels (talkgroups)". Individual sites can be assigned their own System Quick Key, so that as you travel between sites, you can quickly switch between them.