I've had a single engine inboard which was a real crapshoot, but it did teach me to know that slow is your friend, and you need to use the wind or tide. Using the wheel was mandatory on that boat.
My current boat is a dream compared to that one.
When I first got my 300 Sundancer it had two standard rotating outdrives. Backing in to a slip was a nightmare, often made worse by the Dock Mary's who often took the opportunity to point out to me that I had "twin engines", and would start to yell directions. Using the steering wheel was the only way to get in to a tight slip.
Those folks are your worst enemies. What they don't know is that their twin inboards are nothing like twin outdrives that both spin the same way.
A few years back I got myself a counter rotating drive. One of the best things I ever did. It makes me look like a real pro.
If you have twin I/O's that counter rotate, then here is what works for me when it comes time to back in to a slip.
1. Read the wind and/or current. Current is not as much of a factor around here, so I mostly deal with wind. Especially in the afternoon when it can really gust. It's always better to power in to the wind. It gives you more control. Keep this in mind when deciding how to approach a slip.
2. Center your wheel, and drives, as you make your approach to the slip, and then don't touch the wheel again. It's tempting, but it will almost always get you in trouble
3. Use your motors to spin the boat around and line it up. Then while backing in to the wind or current, and anticipating which way it wants to push you, make subtle adjustments using the controls as you back up as slowly as possible.
4. Don't be afraid to pop those controls in and out of gear as many times as you need to. What I usually do is pop a control in to gear just until it starts to move the boat the way I want it to go, then I go back in to neutral and let the momentum move me. When the momentume stops, I pop it in gear again and repeat. This method insures that you are going slow, and allows you to make multiple adjustments if needed.
4. Don't be afraid to abort, and try again. Everyone does it.
5. Try not to rely on well meaning passengers or crew too much. They may not realize that shoving the boat off of something, or yanking it close to something, has an equal and opposite effect on another part of the boat.
When asked by someone what, if anything, they can do, I often politely thank them while asking them to have a seat out of the way until I'm in the slip. A more experienced passenger or crew member generally knows this, and will make themselves usefull by readying a line or fender.
6 Above all: Use the boat as much as possible. The more you do it, the better you will get at it.