1. If Phase 1 is your goal, a Phase 1 mentor is as/more important than a disease site mentor, unless you're in one of those places where Phase 1 is isolated to specific disease states, and not a general Phase 1 program. Bottom line, you need to work with people who know Phase 1.
2. Take
this workshop in clinical trial design.
3. Then take
this course in clinical trial design.
4. Go to ASCO or ASH, or any disease site sub-meetings (ASCO-GI, IASLC, ASBMT, SABCS, etc) that are relevant to your disease of interest, and go to all the Phase 1 talks and take advantage of the mentoring opportunities at all of them.
5. A Phase 1 "fellowship" is definitely not required, but you might find it helpful depending on the resources available at your current program. An away rotation many also be helpful, but can be hard to swing as a fellow.
6. Put together what you've learned in 2, 3 and 4 above (5 if you do it) and write a trial and a grant proposal. Work with the mentors you find in 1-5 above to perfect it. Submit it to every Young Investigator Award, industry sponsor and internal grant mechanism you can find.
Honestly, the connections you make and the work you do will be more helpful to you than being 17th (or even 1st) author on BS retrospective review studies. There are no retrospective Phase 1 trials. If you want to do Phase 1, you need to be all in.