Bullpen sessions both between starts and before starts should always have goals in mind and a specific routine involved. Working with pitchers over the past 10 years and being a professional pitcher myself, it is most important to know yourself. I know this sounds so basic but many pitchers have no idea who they truly are and how their body moves (a feel for the position). Around how many throws it takes them to get loose, what part of the strikezone do they best command their fastball and other pitches and a routine of how to get there. Just like school the more your study and apply your knowledge the better you do.
Here are some basics:
1.) Go through a good dynamic warmup to get your muscles loose and the blood flowing and also some stretch-band work.
2.) Find out around how many throws it takes you to get loose before actually going to the bullpen. Some pitchers like to stretch it out a bit or even long toss before going to the pen.
3.) Work a short bullpen (45-50 ft.) to get a feel for your off-speed pitches before getting on the mound.
4.) Have an on the mound routine. This entails pitches from the stretch and wind-up. Locating all pitches and then situational work. Tailor situational work to hitters you may face that day or in your next start.
If you would like more information on some of my routines please feel free to contact me.