Sometimes it's easier to just start calling them something and then it kind of sticks.

Like Jack-

Vee!

-got his name 'cause Eevee can turn into a whole bunch of things. I think like seven? Seven different evolutions. And they're a normal-type so they can learn all kinds of TMs, too. So he's a jack of all trades. But my starter Susie, I just kind of bounced names off of her and that one stuck.

It gets easier the more you do it.