Sometimes the hardest step is the very first step. And that can certainly be true when it comes to threading the needle. Here are a few tips that have helped me along the way.
1. Cut the floss bluntly at the end with very sharp scissors. Basically you want to ensure that there are no fine fibers sticking out that are preventing your floss from going through the needle.
2. If Step 1 does not work, cut the floss end at a 45 degree angle with very sharp scissors. Then taking that same floss end, pinch it between your fingers and slowly insert into the needle's eye. Sometimes that 45 degree angle is all you need to get that floss to behave.
3. Dab your floss end with a bit of spit (I know it's gross... but it is a tried and true technique). You can also use the tiniest bit of chapstick/uncolored lip balm/beeswax.
4. Try flipping your needle's eye to the other side. There is actually a right and wrong side to your needle. I can never really tell the difference by just looking at it, but I swear that this trick does work.
5. It seems counter-intuitive, but instead of moving your floss through the needle, try moving your needle over the floss. Sometimes that shift of perception does the trick.
6. Try threading the needle over a dark background (for light colored floss) OR over a white background (for dark colored floss)
7. Use a wire loop needle threader. When my eyes get tired or it is really dark outside, this tool is a lifesaver.
8. Put the tiniest amount of hairspray on to the end of your floss. I usually do this by spray my fingers with the hairspray and then running my fingers over the end of the floss. I will also that, for me, this is a method of last resort - I don't want my embroidery to be gunked up with hairspray or to smell like hairspray.
9. This probably goes without saying.... but just to be sure, make sure that you have a needle with a eye large enough for the amount of floss strands you are using. i.e. a sewing machine needle with a tiny eye is not going to be great for a hand embroidery project that uses multiple strands of embroidery floss.