Before going on holidays you can already prepare your skin for a nice tan. The only thing you have to do is to use a scrub which will remove dead skin cells. Like this only your "new" skin will be tanned and the effect will thus last longer.
While sunbathing it is extremely important to use the right amount of suncream with the right SPF! For the whole body you will need about 25 mL in order to make the SPF count. You should use anothe layer of cream every second hour and after swimming (even with waterproof cream!). Between 11:00 AM and 2:00 PM you shouldn't be in the sun at all. I won't talk about the right SPF because it changes from person to person and from place to place where your are staying. Furthermore SPF 30 isn't the same for Europe and the USA so you'd better check the internet for tests that will help you find your ideal SPF. Such things as "sun block" don't exist which means that every sun cream lets through sunbeam which again means that your skin will be tanned with every cream and every SPF!
Make it last
Back home all you have to do is to regularly use a nice greasy body lotion. Aftersun won't be enough to keep the tan, it´s just for the immediate moisture supply after sunbathing. If you don't have a good lotion you can also make a little body mask out of an avocado and curd cheese. And believe it or not but eating carrots is supposed to keep the tan as well!
I hope that this was helpful and I'm curious to know where you guys are going on holidays!