I'm a huge fan of eating out, and London has so many different restaurants and cuisines on offer. Obviously, this all comes at a price (not to mention the effect on your waistline...) but there are ways of eating great food more often, without resorting to the chains (which don't even serve great food). 1. Think local! Obviously, restaurants outside of the tourist trap / rich man's playground of Knightsbridge, Chelsea, Mayfair and Oxford Street are much cheaper. Neighbourhoods outside of zone one are full of cheap, local restaurants, and many areas are known for certain types of cuisine, for example: Shoreditch has lots of vietnamese restaurants, Whitechapel is good for indian food, Lewisham for caribbean food and Dalston/Stoke Newington for turkish food. Lots of competition usually keeps the prices low, and the chefs were usually brought up on the cuisine they're cooking you, so it should be pretty good. (some suggestions further down)
2. Street Food. Okay, not always practical if you want to go out for a "proper" meal with a roof over your head and a waiter, but if there's a big group of you, and especially when the weather's nice, it can be a really fun way to eat. Street Feast London (http://www.streetfeastlondon.com/) has night markets in Dalston on a Friday and Camden on a Saturday. There's about 15 stallholders, which rotates weekly, selling a variety of cuisines, and you can eat very well for £15 tops. Maltby street, near London Bridge is another option, as is Brixton market and Burnt Enz at London Fields.
3. Eat midweek. Restauraters are usually keen to get people through the doors during the week, and so will usually offer deals, set menus, 1/2 price offers etc Mon-Thurs to get people through the doors. At more popular restaurants, this also easier to get a table.
4. Tapas style food: Eat a little bit at home, and then head out somewhere where they serve 'small plates'. It seems to be the rage in London at the moment, so finding somewhere won't be difficult! Obvious options are Polpo, Polpetto, Spuntino, but it's also a great way to try some of the more expensive sushi restaurants.
5. Set menus: An obvious one. Enough said.
6. Google! Some restaurants, even in desirable locations, are crazily cheap! Take Le Mercury (http://www.lemercury.co.uk/), on Upper Street in Angel. Not only is it in a prime location, it's a beautiful restaurant, and the food is great value, with ALL mains at £8.45. Similar offers are available at LMNT (http://www.lmnt.co.uk/) and Little Bay (http://www.littlebay.co.uk/)
7. Deals: By this, I mean deals available through booking on Top Table, or through having a Taste Card or similar. To be honest, I don't usually find them to be great as the restaurant staff can either be a bit snooty, the menu is very limited or the times available to redeem the deal are all a bit weird. But, they work for some people so it's always worth looking to see if there's anything that takes your fancy.
And a few favourites from me...
Huong viet - 12 - 14 Englefield Road, Dalston (no website). Brilliant vietnamese food at great prices. Some of the staff can be a bit rude / ignorant, and the restaurant itself is basic, but it's worth it for the chicken with chilli and lemongrass. Plus it's BYO, which further cuts the price of dinner out.
Rasa - (http://www.rasarestaurants.com/UserPages/index.aspx) mini chain of indian , particularly Keralan, restaurants, with branches in Stoke Newington and Soho. Some are vegetarian, but the food at all of them is great.
Pasta Rustica - 67 Stoke Newington Road (no website). Cheap little Italian, friendly staff, and eating there feels like you're on holiday. Not breaking any culinary boundaries but lovely place. Try the salmon with the creamy sauce and pasta.
The Corner Room - (http://townhallhotel.com/corner_room/) Not the cheapest restaurant, but it is at the Town Hall Hotel in Bethnal Green, the same location as Viajante, which has been widely praised for its inventive menu. The Corner Room is cheaper, but offers a similar type of cuisine, sometimes with dishes that used to be on the Viajante menu.