What factors should I consider when granting an electorate?

Here's what I'd recommend. First, a small country is always good, because you can have a pretty strong hold on them. One province minors are particularly useful. I would recommend you have an alliance and royal marriage with them, because, aside from the relations bonus, alliances give you a +50 score and royal marriages give you a +10 score. Also, you should probably guarantee their independence in order to prevent them from being conquered. Also related to that, try to find a nation that is within the middle of the HRE, nations on the borders, especially the border with France, are much more likely to be picked off. The last thing that can help to make them loyal to you is to make sure they're part of the same culture group as you, which gives a +5 bonus.

Another consideration is beating an OPM out of someone and giving it an electorate - the +100 relations from releasing it in peace deal lasts quite a while and will insure you have its vote for a long time.