To prolong the life of your nail polish; wipe around the top of the bottle with nail polish remover before replacing the lid. This prevents air getting in and causing the nail polish to thicken.
Never thin nail polish with nail polish remover. Most nail polish remover contains oil which can prevent the nail polish from drying properly and or cause it to chip and flake. Always use a specific nail polish thinning product or a few drops of pure acetone.
To remove stains from nails soak a cotton pad in a little pure lemon juice and place on the nails for a few minutes. Prevent staining by always using base coat under strong colours. If you have nail discolouration below the surface of the nail this may be caused by damage to the nail bed or a fungal nail infection. If in doubt consult your GP or pharmacist.
If you suffer from flaking or weak nails try replacing your usual nail polish remover with an acetone free version. Acetone is a key ingredient in most nail polish removers. It is dehydrating to nails and can exacerbate brittle and flaking nail problems. Cutex Acetone Free Nail Polish Remover (200ml £1.99 Boots.com). A good quality nail hardening product can also help with this problem such as OPI Nail Envy range or Sally Hansen nail hardening and strengthening products.
Always file your nails in one direction only. Seesaw filing action generates heat and causes friction which can damage the nail and lead to flaking and breaking.