Don't be fussy
I have such an urge to see every city and town in the world so I plan one holiday a year to tick something off my bucket list. Then for mini trips I simply book whatever is the cheapest flight. If you don't have a city in mind, I tend to go on Ryanair Farefinder. This way I've managed to bag trips to Cologne and Derry both for under £20 return! Simply select your nearest airport, click to go anywhere and set the budget to under £20 to see what comes up. You do have to be quite spontaneous but you can book quite far in advance too. I booked my trip to Derry in January last week! Be adventurous and go somewhere you never thought you would!
Go out of season
For our big holiday I have to go during school holidays as my best friend is a teacher. So selfish eh? But I try to go early spring or in the winter for my mini breaks. You'll find cheaper flights and accommodation as well as beating those pesky tourist crowds.
Rent an apartment
I am a huge fan of Air BnB, having used it numerous amounts of time. I just love staying in someone's house in a local neighbourhood and feeling like it's my city for a few days. The hosts have always been charming and so helpful. You can find some great deals and it really does help to keep accommodation costs to a minimum. Make sure you have a proper scroll through the apartments to ensure you get the best deal - don't just look at the first couple of pages. If you really want to save some cash, have a browse of the rooms in someone's house as they are even cheaper still.
Go midweek
I know some people want to maximise their holiday and go Friday to Monday for a weekend break but this is often when flights are expensive. Obviously if you book far enough in advance they can be cheap but that involves serious organisation. At the minute I'm favouring a break from Wednesday till Saturday. That way you have the rest of the weekend to recover as there's nothing I hate more than going into work the day after a holiday. I need time to readjust to my surroundings! We always take a three night/four day city break. I like to have a couple of days hardcore sightseeing then a day just mooching around without any plans - that way you get a real feel for the city and see how the locals live. My favourite times in London have been spent aimlessly wandering around!