A few things that I have always used (many covered in the 6 tips posts)
1. A well anchored IV attempt will have a much higher chance of success.
2. If you are starting a line in the AC, aim low, feel where the vein tracks below where you see it in the fossa and poke about a fingerwidth below it.
3. IV catheter bending - I learned it from anesthesia - if you have the BD Insyte catheters, when you remove the cap, use the inside of the cap to bend the needle slightly. This allows a more shallow angle and a more direct approach, especially on AC and EJ IVs.
http://www.youtube.com/watch?feature=player_embedded&v=0LT47_R5r_M
This video shows what I am talking about, for regular veins, I bend it only slightly, but I feel like it makes a difference.
4. If you are having trouble finding veins in fat people, grab a blood pressure cuff, inflate it to half way between their diastolic and systolic blood pressure and use that as the tourniquet. Oftentimes the regular tourniquets just dont work well on fat people.
5. Find your sweet spot - Over time, you find a spot that works really well for you when you start IVs, that is always the first place I look. I always look at the top of the hand first, then the forearms, radial, then the AC (selection varies based on ease of placement and timeframe the line is needed in).
6. Practice makes perfect. Go to a place like ambulatory surgery where they are doing a ton of IVs every day and get some practice there. Otherwise talk to some paramedics, paramedics are IV fanatics and oftentimes have very good skills due to the volume that they place in the field.