As for forced movement I do this:
1. you get pushed into a square with no ground into it.
2. you make a saving throw to not be pushed. If you succeed you are squeezing. If you fail you are no hanging off the edge and you are prone. While you are hanging you make a increasingly high endurance check to not fall off everytime you are hit. You are also immune to forced movement.
3. while you are hanging an ally can make a minor action athletics check to pull you up.
4. you can get up yourself with a relatively easy athletics check as a move action.
This is a perfectly valid option if you value "balance" highly - but I like my rules to be reasonable in the sense that they don't undermine suspension of disbelief.
And these rules may be balanced, but they're pretty unreasonable. There are preexisting rules for dealing with forced movement into hazardous terrain (a nebulous concept in the first place, to be sure).
Why would you be squeezing if you succeed on the save? There might well be plenty of space - it makes no sense. The PHB rules suggest that you succeed in dropping prone and avoid being pushed off, which is far more logical.
It makes sense sometimes for a character that has been pushed off a ledge to be able to grasp the edge and remain hanging - but that's not necessarily an easy feat, and it really shouldn't be automatic. It makes even less sense for you to be immune to forced movement - after all, you're just barely hanging on, all the opponent has to do is stomp on your fingers - or cut them off, say. In RAW terms, you might rule that they're helpless.
A minor athletics check is far, far too easy to let someone climb back up. Two characters are spending time trying to get the character back on his feet, and that should be reflected by a much greater action expenditure than a mere minor action for one of the two character. Perhaps a standard action on the side of the helper, and the hanging character ends up prone adjacent to the ledge (i.e. they need to expend a move action to stand up later). Depending on the game style, you might make it a move action for the helping character, and you might or might not make the action provoke an OA.
Of course it's possible to tweak the rules to make a cliff a non-threat - but you run the risk of making the entire situation ridiculous, and what's the point of the cliff in the first place, then?
When push comes to shove (no pun intended), I'd rather have an unbalanced reasonable approach than a balanced unreasonable one, particularly since it
is indeed possible for the DM and the PC's to indeed avoid adventures in such areas at low level. If necessary, tell your PC's that it's just a matter of time until some suicidal orc simply one of them off a cliff if they insist on picking fights next to a high cliff, and make sure the campaign has options to avoid such fights - say, by simply starting the combat at least 6 squares away from the edge, so that a single lucky forced teleport or move cannot push someone over.
