One more note on "location" farming. Look at creep levels when you are ready to choose where to farm next lower level creeps usually die faster meaning fast gold faster xp often time getting you 1 more upgrade to attack the next spawn with.
If you are melee and don't have orb yet try to avoid a spawn that has 3 flying creeps unless you have spells to kill them.
If you are heavily dependent on spells don't run into a camp with 5 infernos unless you want to take 5 hours killing them, go kill another couple of areas get a bit stronger then take out that camp or have a teammate that is dps take them out
Try to farm in patterns for example middle bottom front bottom back heal top mid top front top back (that would be a pvp pattern adjust per team size). This will minimize the amount of time running from spawn to spawn and from spawn to base.
Save this link for a replay to see Carefulibite's tips in action
http://blinkgaming.net/replays/GHost%2B%2B%202012-07-06%2021-56%20custom%20hero%20arena%20tutorial%20%2835m09s%29.w3g. Download the file then search in wc3 folder to view replay. Pay attention to how team 1 farms. Not a perfect example but enough to see what a difference it can make.