I realised recently I have a set rules / patterns I observe when choosing T-Shirts to buy. Here they are:

1. No slogans, especially funny ones. It's not the 80's any more.
2. Avoid logos. I'm not a walking billboard.
3. Don't buy ringer-less white cotton - they always look like you got it free with 10 beers, whatever's actually printed on it.
4. I don't like designs with straight edges, imprecise edges helps it blend with the rest of the shirt and look much classier, as does having the print somewhere other than the centre of the chest.
5. Don't shop where everyone else does. The internet helps here - here's my shortlist of favourite suppliers. NB however that American sizes are sometimes one up from UK ones - get measurements if you can.