I spent a few years in the tropics and the advice given to me was to have a hot shower an hour or so after you come out of the sun - about as hot as you can stand but obviously not so hot that it's painful. It makes the skin sting for a few seconds but then settles down. Lots of soap or shower gel.
Then dry off and put on some after sun.
Do that for two or three days and the tan really starts to develop.
I think the secret is A) Don't overdo it and burn, and B) keeping the skin hydrated (hence the shower, etc)
It helps to start off in the morning, say from 10 to 12, using a decent SPF (15 or so - more if you're very light skinned) and keep moving!! I mean 20 minutes face down, then turn over for 20 minutes, alternating between head towards the sun and then feet towards so that you tan fairly evenly. If you get too hot go in the pool or the sea for five minutes.
Also, its sensible at first to stay out of the sun from Midday through to about 2:00 which generally co-incides with lunch and a drink anyway - that's the time many people get burned.
If you feel up to it, maybe another hour in the afternoon although at first you're better off sitting in the shade under a parasol/beach umbrella - remember that a sandy beach or paved area around a pool actually reflects a lot of sun back up, so you can burn even if you think you're in full shade - keep putting on the SPF!
After a couple of days reduce the SPF factor but don't ever be tempted to not use anything
It works for me - my partner gets indignant at the fact that after 3 or 4 days I look like I've been there for months!
And of course the occasional cold Keo helps.....:-)