I'd look in the FAQ as there is a detailed description of my recommendations during the job search. However, during residency there are a few things you can do to max the chances of getting a good private practice job:
1. If you want to stay local, get to know the local radiation oncologists. In the course of residency, private practice physicians will often refer complex cases to your academic Medical Center. Some would characterize these referrals as "dumping." Regardless, this will let you identify local groups and also to interface with physicians directly on the phone. During residency when I spoke to referring private practice radiation oncologists they would sometimes ask me if I was looking for a job or if I knew anybody good who was. This really represents the perfect opportunity to interface.
2. If you want to practice in an area outside of your residency then it is important to keep in touch with your co-residents at other institutions. This will be a slight conflict of interest so you should proceed carefully. However your co-residents will be the best people to talk to to help identify local private practice radiation oncology facilities.
3. Diversify your training as much as possible. The more modalities you know the better off you will be from a marketing perspective. SRS, brachytherapy, protons, radioisotopes, and experience with different treatment planning systems will not only enrich your training but will make your CV a good sell for private practice jobs. If your residency program does not offer a particular modality then you would do well to do an away rotation to learn it.
4. Make friends with everybody. Radiation oncology is a small field and gossipy one. In addition to your letter of recommendation, potential employers will also solicit opinions from people they know in your department. Therefore if you piss off somebody, it could cost you a job.
5. Research during residency is a bit of a wash with private practice jobs. However, in my opinion, any private practice group who is intimidated by your publication record is not a group you would want to join. In fact in many of the more prestigious private groups, a good publication record is viewed favorably.