My most important rule and one which I rigidly adhere to is never to pay for entry to a competition.    I also avoid gambling sites, including bingo sites, like the plague.

While many competitions are used to harvest e-mail accounts, it is prudent to keep a separate e-mail adddress for entering competitions

Using a form filler for your entries saves a considerable amount of time.    I use last pass which is free and easy to set up and use.

Some of my favourite sites are;

ukhotdeals
ukwins
loquax 

While yougov is not strictly a competiton site, points are awarded for completed surveys.   These points can be saved for cash or entered in the monthly prize draw.   This is one of the better survey/prize draw sites and I strongly recommend it.