Ok I'm going to say it once. If you are farming by yourself and UNLESS your tp is on cooldown (yes if you are farming by yourself you better be carrying a tp), then you have the additional responsibility to check the minimap every 3 seconds and tp to the nearest tower as soon as you see a gank going on (a gank meaning a group of enemy heroes going after your allies, more specifically when the total number of enemy heroes is greater than your allies by one). If its like four enemy heroes ganking on one hero, then unless you know your friends are coming to tp as well, then don't bother.
This takes practice because you need to tp as soon as you see the enemy heroes coming, its too late if they already unleashed their spells and your ally is yellow/red hp.
I admit I don't do this all the time too but at least I try.