As far as the question of protecting the archer/cleric/etc from bad guys goes, it's usually possible.
I'll grant that it's quite difficult when you're facing a party of rogues. It's quite difficult to keep a group of rogues from doing anything. However, when facing groups of fighters or groups of monster, or spellcasters using touch spells, it's quite possible to protect your fellow party members.
Most of the time, you can interpose your character between the villain and the characters to be protected. That's usually enough. Most bad guys don't want to risk AoOs, don't have tumble, and would rather make full attack actions anyway. Sometimes they do risk the AoO though. That's when you trip, disarm (a favorite of mine--even a middling BAB works wonders with a glaive against one handed weapons

), or grapple. You can also let them take their one swing at a spellcaster and move to flank them on the next round. That works really well if you're a rogue.
In other cases, you have to set it up so that you block the only available straight lines between the villain and the person you're protecting and the distance is too great to move and attack. Alternatively, you can arrange your characters so that right next to the archer or spellcaster would be a really really bad tactical position for any villain (it would just be a five foot step for the major damage dealers and the rogue to all be flanking or something like that).
Even so, there are no foolproof methods for protecting another character. Just like there are no foolproof strategies for taking out bad guys or anything in D&D. Part of the fun is not being able to control everything. . . .