1.Do you trim wet or dry?
I trim my hair when it's dry if I'm only doing a microtrim, which is my usual method. If I'm taking off more than half an inch I might trim my hair when it's damp.

2. Any particular kind of scissors that work better with hair?
I use the fancy Mizutani shears that I got with my kit from beauty school, but it's not necessary to have super-fancy, super-expensive shears just for trims. Before getting my Japanese shears I just used a $40 set from Sally's.

3. How do you get it straight at the bottom? (what different methods are there for different legths of hair, I imagine that people with really long hair have an easier time with this, am I right?
I use Feye's method for trimming straight across, like a lot of the others here.

4. Do you have any other personal tips & tricks/ favourite methods, please share!
Trim your ends in sections, so you're not cutting into too thick a chunk of hair right away and fraying your ends/dulling your shears. Also, only take off a tiny bit at first, then take time to check the length and see how your ends feel. You can always trim off more if you want to. And whether you're trimming length or just individual splits, trim in bright light!

5. Are there anything one shold absolutely avoid when trimming yourself? Any absolute No-No's?
Don't trim when you're feeling really angry or frustrated with your hair, wait til you're feeling calmer.