When fighting other casters distance can be something of a disadvantage because you can not use some of your Eve isk abilities at maximum range. Try to stay within medium range so you can use spells like fire blast and counter-spell.
Of course you will never win a fight by running away. Knowing how to DPS is the second part of mage PVP. This will depend largely on your spec, but you will usually try to use your fastest and hardest hitting abilities.
Arcane missile is a PVP favorite because with talents it can not be interrupted and deals great damage. Try to save it for times when a melee is beating on you and you can not get away.
Scorch is another good spell to use for repeated DPS because it can be powered up with talents and has a very short casting time. Use it on enemies that are almost dead and running away or not focusing on you.
Frost bolt will usually be your main nuke of choice though because of its chill effect and relatively short cast time. This is especially true if you PVP as a frost spec which is considered the best PVP spec.
Pyroblast are another great PVP ability, but only when combined with presence of mind or you have Polymorphed your opponent and he can not do anything.