I personal don't use elective anymore since 2.5 because it got much harder to get the right person on the throne.
I usually don't have problems with it, but only if you can get it to work to your advantage. There's several things I do:
1.) If there's a candidate with enough votes you like, and if you're depressed, then commit suicide! Do it before someone changes their mind.
2.) Revoke a title from an unhappy vassal to steal a vote from them (assuming they have only one demesne). It gives about -15 vassal opinion, but I find it's not enough to sway other votes away from your successor.
3.) Content characters are more willing to vote for your chosen successor, since they naturally will have high opinion of you. This means giving titles to mostly Content characters is best; however, ambitious characters aren't necessarily bad either, because they'll tend to vote for themselves, meaning they're not as likely to be deciding votes, since they'll waste one vote on themselves, even if no one else votes for them.
4.) Have all landed characters be dynasty members, so no matter who gets voted in, you'll control the ruler. If a character you hate wins the election, find a way to get him or her killed off asap. Have him lead armies, regardless of his military skill. Ignore assassination plots against you. Pursue depression acquiring events, such as Stewardship focus and the Falconer event. Definitely revoke a title or two with this character, so people hate you more, to increase the plot power against you. Assign a Spymaster to your Council who hates you.
5.) If a character you hate is winning, either Spy on them to spread false rumors, find an excuse to imprison them in the oubliette (-10 diplomacy may convince others to stop voting for him, while the -1 health might kill him off so you won't have to), or assassinate them. Keep in mind if they're a dynasty member, you'll become a Kinslayer, so assassination isn't recommended. If you can't perform these actions on the leading candidate, instead, target the people supporting the candidate.
6.) People are more likely to vote for your candidate if you're well-liked. This requires all my leaders to have high diplomacy and lots of carousing (I like to rush for its diplomacy lifestyle trait). I love the Attractive trait on female rulers for the flat +30 attraction modifier towards all males, making espionage missions easier, since it's mostly men that drive the engine. Furthermore, striving for rulers with high diplomacy makes it easier to get them elected anyways, because it's high diplomacy leaders who are more liked by their peers, resulting in more votes for them.
7.) If all else fails, force a succession law change to something giving your chosen heir the keys to the kingdom. You can always switch back to Elective once that ruler has governed for enough years. Problem is you'll get opinion penalties for succession changes, but your citizens will just have to deal with it.
