We sold a few over the years, my advice would be to put it up for more than you want for it (people like to get a bargain and offer less), check out your estate agent - ring some of them up and see if they answer their phone!!!! Look at their other properties and see how well those are photographed and whether they sell or not in the time you are considering putting yours up for sale. NEVER sign anything without reading the small print (e.g. the full fee shall be paid if the vendor sells the property even if we dont introduce the client blah blah). Walk away from anyone who pressures you - tell them you need time to think about it before you sign.
After that its the usual advice give it good kerb appeal, it does not cost money to keep it tidy and smelling nice with a bit of a de-clutter but dont make it look clinical, it should still look like a loving home. We have had various pets over the years and its never put anyone off as we have always maintained cages and washed pets that were smelly. If you have pets its important to get that vac out (and pop a freshner in it).
Its best if there is a job that needs doing if you can afford it to get it done, as people assume it will cost a fortune to get any job done, and if they have a choice of houses they will usually end up going for the one that needs the least work.
Cost effective ways of making you house look better - buy light coloured curtains and bedding (they look fresher and cleaner and bounce more light around rooms), re-emulsion any paintwork that is looking drab and hire a carpet cleaner if your carpets are looking a bit dowdy (some of them have attatchments that can do your sofa too). You could also buy a rug and cushions (and remember you take your bedding, rugs and cushions with you so its not "lost" money).
Lastly, try to change any really dark decor - even if it means painting over wallpaper - some dark wallpapers really make a room seem dark. Just remembered we had a friend years ago who pulled up a carpet in his new home to find a witches pentagram painted onto the floorboards! - now that is mega dark!