Easy stuff.
First off, you want to save all your minecraft data that you use for the server, just cut and paste it somewhere else so you can switch back and forth from single player to multiplayer.
Next, you want to find all the addons you want. They're all over the minecraft forums along with planetminecraft.com(net?) I just google them myself, some google terms that may be useful are "Minecraft World edit" "Minecraft too many items" and I don't have one that makes you fly however I'm sure they're out there. Caution when using any mod that adds items, only painterly texture packs are really compatible with those and I'm talking about only the really well know item add ins (Mo monsters, millinaire, ect). You'll also probably need modloader which is another plugin easily found via google.
So, you have your original minecraft data stored, you'll lose all your multiplayer stats, including achievements, for single player mode unless you dig through all the files and only transfer the ones you want, which is just a massive hastle especially if you want to switch back and forth. Now what you want to do is start minecraft, login and get a fresh new minecraft install.
After this comes the tricky part. If you don't have 7-zip, you'll want to download that so you can open the minecraft.jar, once you have it opened, go ahead and take any of the mods you've downloaded and add them into the .jar. Before you exit, you'll want to delete the "META-INF" file from the .jar (don't know why but all mods tell you to do this).
Finally, close the .jar and you should be all set. A couple of tips you may want to hear are: Be careful. Lot's of new mods come out and have virus's on them. Backdoor traps, trojans, keyloggers, you name it you can probably find it in a random minecraft mod. Scroll down and look at comments for the mod itself if you're downloading it from the forums, I generally go in and look through 3 random pages just to make sure it's not the same guy posting over and over again "Great mod! Hope you update it soon!" and all that mumbo jumbo... If it's fake you'll usually find someone saying "Thanks for the virus fucktard" or something like that. Obviously.
Anyway I hope this helps, if you have any questions feel free to reply, I may have missed something considering I worked and then walked 2 miles home but this should cover most of it.
Enjoy!
