I've never had a tan in my life and heard my friends banging on about how going on a sunbed would help. I went on every day for two weeks before on a two week beach holiday (stupid, i know, but I was only a teenager at the time).
I went on the sunbed and tried accelerators (which left my skin red raw and feeling like i'd been burned). Went on holiday and fried on a sun lounger... and came home with more freckles but my usual white skin.
Worryingly, I developed a black mole on my leg a couple of weeks later. My mum's a nurse and ordered me to go to the doctor - the day before I shaved my legs and took the mole off! Owch! It never grew back, thank God, but suspect it was a dodgy one.
I've just had to accept I'll never get a tan and with my skin colour trying to get one will be fruitless and will do me serious damage. If you don't have a high amount of melanin in your skin no amount of accelerators or sunbeds (or sun) will increase it.
I now whack on the factor 30 in the sun and use a good self-tan in summer (the best are St Tropez for a dark tan, Elegant Touch Crackle for a medium tan, and Asda Sun System for a paler tan that you can build up - very cheap for under �3 a tube and it never streaks).
And you can be happy in the knowledge that you won't age as quickly as your friends - I'm in my 30s but look five years younger while all my sunworshipping friends are getting wrinkles already and look older than me!