Here is some things that have helped me for suicides (written for right-handed people). I think they are one ofthose things that aren't as hard as they'd seem at first, but seem impossible until you actually get to that point...
You want a fairly new string. I've found that old string has a tendancy to stick together and so is a lot harder to work with. On that same vein, make sure the string is not tight or loose, since it'll want to wrap around itself. Especially when I was learning, I had to check the tension a lot. Usually I let it hang down, then bring the yo-yo next to my yo-yo hand and see if the string wants to twist around itself. The tension can change a lot if you mess up, and sometimes even if you don't, so keep an eye on that.
You want light behind you and dark in front, since it helps a lot to be able to see the string clearly.
One big key is that for your right hand, you want to keep all your fingers extended like a relaxed type handshake. Then when you start to swing the yo-yo around, you want the loop of string (the double strings that are around your left hand index finger) to actually bend around your pinky finger. Usually is good to have it hit at around the midpoint between your left hand index and the yo-yo.
Then as the yo-yo is almost at the top of its upward arc (but not going to the left yet), only then let the loop of string slip off your left hand finger.
That bending of the loop around the fingers of your right hand before it is released seems to help open up the loop of string...
Also, I generally keep the right hand in the same place, and do the swinging of the yo-yo with my left... I found that moving the right hand around a lot can mess things up.
If you can get to the point where you're seeing the loop flying around, open enough to put your finger in, but are always reacting too late so that you miss, that is a good sign... =)
After that, it is more a matter of learning where the loop is going to be based on how you swing it and anticipating. Also at that point, you probably want to look down at your hands less, and more upward at where the loop is going to be...
Experiment with the speed of your swinging. You can swing it a lot slower then you'd think, and that can help a lot, especially at first. Try swinging it slower then you think you should, and you'll probably be in good shape.
The speed and lots of other things also effects how long it takes for the loop to open up, where the loop is open the most, etc. After a while you may be able to control it so that you catch the loop further to the left (can look more impressive), or pretty close by (better for doing multiple suicides in a row).
The main thing is to keep at it. At first I thought I seriously questioned if I'd be able to do suicides, and now I'd like to think I'm good at them, so there is always hope...
Also, try looking at my videos of the suicides, and videos from other people. Maybe be able to glean something helpful...
A couple of other notes. You may want to experiment with string length. You would think that a longer string would help since the loop is bigger, but it also seems to want to stick together and not open up quite as well. Going a little shorter may be good to try. Normal string vs. slick 6 or slick 8 may yield different results. I haven't had a chance to experiment too much with that, though.
Also, sometimes you see someone putting multiple fingers into the loop before doing the suicide. While I am sure that it can help to a degree, I'd recommend against it in general, just because it interferes with speed smoothness. It is hard to go straight into a suicide from another trick (or multiple suicides in a row) if you have to stop to stick more fingers in the loop.
Good luck. I hope that this guide helps... =)
