Choose a locomotive that runs well on conventional DC power.
Digital decoders cannot compensate for faulty motor operation, poor track pickup, etc.
If you are not happy with the way your locomotive runs on regular DC power, installing a decoder will not make it run any better.
If there are any mechanical issues with your locomotive, fix them before you install the decoder.
Since you have to open up the loco anyway, do a tune up to get it running really well before you put in the decoder.
Digitrax recommends using a conductive brush lubricant like Aero Car Technology's "Conducta" brush lubricant to minimize brush noise in all locos.
Be sure the brushes are making good contact and that the commutator is reasonably clean.
Decide where the decoder will fit inside the loco.
Is there space to put the decoder or will you need to "make room?"
Is there a plug ‘n play decoder for the loco?
Digitrax offers a variety of decoder sizes, form factors and current ratings to accommodate almost any locomotive.
Decoders from other companies that make compatible DCC decoders may fit better in some of your locos, too.
If there is just nowhere in the locomotive to install a decoder, you can run it on your Digitrax system as an analog locomotive on address "00", although do not recommend that practice because of heat build-up when operated in this manner.