Multiple ON yomi Kanji - What To Do?
Here is what you do: "Create" the memorable element(the kanji) in one location (Which corresponds to only ONE reading), and then you take that same element, and place it in another location, making sure you're aware that the element/kanji is in more than one spot. Then when you see the kanji in an unknown word, hopefully you'll remember that there are two ways of pronouncing the kanji, and hopefully you'll choose the right one. If you forget the reading, go back and recall what role the kanji played in a particular location, always doing your best to associate the particular atmosphere of the location with the kanji. You're not going to forget the reading for quite some time (Location is a strong memory aid), but make sure you somehow review that kanji and reading some time in the future and enforce the memory aid, otherwise you will forget the connection like you would anything else you don't review.
If you choose the wrong reading (Meaning you knew both of the readings, but chose the wrong one), all I've done is make sure I know the correct pronunciation of the word, then review it several days later. If I recall the reading of the word several days later, it's a nice 成功 (せいこう) :)
Let's take the example of "祝." This corresponds to しゅく and しゅ. I know that in しゅ (The location which corresponds roughly to a war-zone), that teenagers gather around a certain location and celebrate the fun they are having in the battlezone. I see that image in my mind, and then I recall that in しゅく (The location which corresponds to an elaborate amusement park ride) is where a group of teenagers go to celebrate their being on vacation (Assuming that's why they are at such an amusement park). The manner in which the teenagers celebrate in the amusement park has to look very similar to those in the war zone. I'll compare the two images and "look" closely at what the kids are doing. The kids in the war zone are shooting their guns in the air because they're so happy, similar to how the kids in the amusement park throw their hats and such in the air. If I "created" the amusement park location first, I would make sure that what happened in the warzone mirrored what happened in the amusement park. Once this is done, all you need to do is remember this connection a few more times and you're one step closer to Japanese literacy!
If you have further questions, comments, etc. feel free to ask in the comments section :)