I do think of names in advance, but I wait to name the fish until after I get them. I haven't used any of my pre-prepared names yet. :P
For example, when I brought Robert home I looked through my list and then suddenly decided he needed to be named after a dead arctic explorer because of his white lips. xD And then the rest of my fish became the other deceased members of his exploration party.
LOL!!! At least the crew gets to live on in spirit through your fish xD
When I got my betta's I chose their names in advance because I wanted them to be strong. I was following a saying I heard once: "If you give something a name you give it strength and power." I wanted them to be strong and healthy so I named them Kaida for Little Dragon and Kalona after the archangel from the House Of Night books. Their strength was proven after they fought so hard to keep alive and come home with us. Even though they didn't live long Kaida was a fighter until the very end.
My next betta's will probably be girls so I've been thinking about strong names that would fit them. Such as Sol for Sun or Sonrisa for smile. Enrico for a boy would be fitting...I think I'm going to let my heart decide next time though. Thinking about names however isn't bad.
When I got my first betta, Finny, (r.i.p.) it took me awhile to name him, so I just named him Finny. I hope to get a sorority soon, and I want to get a pink-ish female and name her Euphemia :D (can anyone guess where I got the name?)