An important thing to note is that lately a lot of DPS stations have been converted to P25. In the past few months, both DPS Houston and DPS Conroe have switched to digital. I don't know how many others are planning to switch this year, but DPS Conroe just switched over on the 12th. DPS Houston switched over last December. It appears that the process has gained quite a lot of momentum, and more may be switching over in the next few months (again, I don't really know who).
If you are travelling through Waco, you should be able to pick up DPS Waco (KKC523) on 155.460 (PL162.2), as well as DPS Austin (KKD303), also on 155.460. If you're lucky, you can even pick up DPS Bryan (KKV232) on 155.460. Note the callsigns. These are important when listening to DPS, for example (base side only)
Bryan, 6519...
6519, 10-4, KKV232.
DPS uses a duplex system, so unless you're really close, you won't pick up the unit traffic (on 154.680). All you'll hear will be what was stated above in the example.
There is an excpetion, though. Some stations have a repeater on 159.210 (PL varies with station), where you can hear all of the traffic. I believe that DPS Waco has their base freq (155.460) repeaterized, so you can hear both ends of the conversation.