The following is all IMHO, and whatever works for you is obviously what you should do!
Patrons are supernatural creatures which will bond with a wizard or elf because it serves their interests. Wizards and elves may bond with these beings because their interests are served as well. It is a semi-symbiotic relationship, but it is one in which the patron usually holds the trump card. For everything the patron does for the mortal, the patron will expect something in return. This is in addition to the effects of patron taint, which can be severe. Yet acquiring a patron is the road to true arcane power, for only through a patron can a mortal hope to wield greater-than-mortal powers.
A patron should be a source of spells, and a source of information on where to find spell formulae. But the patron is also a source of adventures based on its demands on the wizard or elf. The relationship with the patron should be at least as much a matter of role-playing as it is one of rolling dice. The mortal should wonder whether or not taking this particular patron was a good decision or not. Sometimes it should seem a good decision. Sometimes it should seem a real burden.
There are a lot of examples of this type of relationship in Appendix N fiction -- not only the heroes who take patrons, such as Elric, but also the villains who do the same. If you can find Manly Wade Wellman's Who Fears the Devil?, there are some excellent (and creepy) examples in that book. Even nominally "good" patrons, like Ardneh in Fred Saberhagen's Changeling Earth or the Silent Ones in A. Merritt's The Moon Pool, are willing to sacrifice their pawns if they need to in order to win the game.
Don't forget that, for every favour done, the patron gets a marker to call in from the caster. This means that the judge can send the caster on quests that the caster might otherwise prefer to avoid. Want to run Raggi's Death Frost Doom, but feel certain that your players would never have their characters enter the petrified cabin? When the wizard's patron tells him to go, you can feel certain that at least one of the players is committed. And how you play their involvement depends upon the patron. Some patrons may wish to keep the status quo, sending the PCs to stop an NPC party. Some may wish to recover something without disturbing the dead. Some may simply sacrifice the caster in order to overrun a town with an army of darkness.
You might also want to take a look at how patrons are described in DCC modules. In The Portal Under the Stars, the goat-man is looking for a tool on this plane. In The Emerald Enchanter, the patron helps the titular miscreant chain various demons for his experiments. The patrons described in The Ooze Pits of Jonas Gralk and Perils of the Sunken City are ones most mortals would consider it wise to avoid -- in Perils, the ruined area is ruined because a patron felt insulted by the caster taking on a second patron!
Don't even get me started on The Balance Blade, although you can be damn sure that this module is entirely in keeping with the sort of things that occur in Appendix N fiction. I intend to run it in my regular campaign, with the only change being omitting the Will save at the end -- the player can choose to act as his patron demands, or sever the relationship and earn that patron's enmity.
The judge should keep a patron's personality in mind, and try to ensure that the player or a wizard or elf is constantly reminded of who his patron is. A player who chooses a patron on the basis of spells, invoke patron results, and other tables without actually looking to see how the patron is described is making a mistake. Serving Moorcock's Arioch should not seem like serving Lovecraft's Nyarlathotep, and neither should seem like serving Wellman's One Other, if for no other reason than that they present themselves differently and have different goals.
The more the caster serves the patron's agenda, the more the patron is willing to do for him. This might include things far beyond the mere spells and invocations described in the Core Rulebook -- again, let Appendix N fiction and modules like The Emerald Enchanter and The 13th Skull be your guide here. Even a relatively benign patron, like Radu, King of Rabbits, has an agenda. Serve your patron well and you will be rewarded. Depending upon the patron, that might be a heartfelt reward, or it might last only until you are no longer useful.
Patrons allow you to do great things. But when you have a patron, you are no longer entirely your own person. There are pros and cons to both having a patron, and shunning patrons altogether. If a mortal is going to seek a patron, he should be wary about which patron to accept, for there are many more bad deals than good in the shifting planes of the cosmos. Many crafty and cruel beings harken to the first call of a mortal seeking a patron, and they will take much in exchange for little. If your casters are ever entirely certain that they have chosen wisely, then you are far more lenient than I am!
Cathbad the Meek (herbalist Wizard 1): AC 9; 4 hp; S 7, A 7, St 10, P 17, I 13, L 8; Neutral; Club, herbs, 50' rope, 50 cp; -1 to melee attack rolls. Hideous scar.