Couple of tips:
Don't do something with 'Hit the North 2010' on or similar; the t-shirt then becomes dated and looks a bit shit next year etc.
Don't do something with thousands of sponsors' logos all over it; looks confusing and messy.
Just a simple logo, done in a way so it looks 'cool'. Nice colours, Decent quality fabric. Something that someone would wear in a range of situations, not just at a bike event.
Don't make it tooo bikey, or it becomes a bit 'trainspottery', you know?
Something a person would happily wear, like they would something they'd go down the pub/to a club in.
Avoid the 'given free with entry' type cheapo tatty affair.
'Oh but I paid £4 for entry, and got a free t-shirt'
Yes, but you also wear shiny running shorts (they had a pant liner, but it perished), and you smell a little bit funny…