It's strange how changing the position of your legs can effect the difficulty of the exercise. Here try this: Do a pull-up with your legs bent behind you, easy? Now try it with your legs extended forward about 35 degrees perpendicular to your body.

You'll find a big difference! What this does is by you changing your center of gravity you're also changing the leverage. With your legs extended forward you are taking your leverage away, by bending them backwards you are giving yourself more.

For future reference you can make a lot of exercises much more difficult by reducing the amount of leverage.

As for pullups you can do wide grip (palms forward) to put more emphasis on your back muscles, or do a regular chin-up which stresses more your biceps and shoulders.

I try not to use my momentum on a pull-up because it requires less strength for instance look at how the powerclean movement is performed using the momentum of the legs to get the weight at shoulder level. On the flipside I like to do it at a fast pace without advantage of momentum, and using my center of gravity to make it harder.

You can also keep track of the intensity of the exercise. # of Repetitions over time. Another factor you can throw in is frequency (how many times you perform the set throughout the day) and variation of the exercise.

You should do the harder variation first so that you'll be able perform more repetitions. For example with the pull up you do a set of the wide grip first, then you do the easier regular chin-up.