There's no one 'trick' to solving creative briefs and coming up with good advertising ideas (sorry!), every problem is different. But there are things you can do to help things along. One of them is very simple - stupidly simple in fact (my favourite kind). Talk to people.
Hardly rocket science, but it is something that can be forgotten too easily. A lot of creatives think that you should take the brief and go and lock yourself in your office and not come out until you have 'the idea'. But don't underestimate the effectiveness of just talking to people about the problem you are working on. It might be seen as a bit old fashioned in a world where the modus operandi of a lot creatives seems to be 'get brief, go on computer' but I guarantee it will help a lot.
It's obvious that that's one advantage of traditional agency creative teams - having someone to talk over ideas and the problem with on a permanent basis is massively more helpful and productive (and fun) than sitting on your own in a room waiting for inspiration to strike. But look further afield than that. Keep talking to everyone involved - talk to the account director about their perspective, ask an account manager what they think about it, go and bug the planner - ask them some difficult questions. You should always be talking and questioning everyone on the project, find out what people really think. More importantly you should make sure you are talking to your client - before, during and after the briefing, go down to their office/factory/whatever - ask them awkward questions that no one else would ask, find out what they think the real problems are. You'll be amazing how many important and potentially massively helpful things don't make it into the brief.
Also, make sure you talk to as many people from 'the real world' as you can about the product you're working on, ask your mates, your mum, a cab driver, the man you buy your newspaper off. A LOT of very helpful things, insights and opinions can lurk in these conversations.
So that's it really. Don't feel like you have to sit on your own in a quiet room to come up with an interesting idea. Go outside.
Have a chat.