Tough question, Calvin L. Todd Jr.! In order to best answer it I'll make some assumptions. I'm assuming that you are looking for entity selection advice specifically relating to private investors investing equity and/or debt. If I'm off track let me know.
First off, I'm neither a lawyer nor an accountant. Just an investor that has been there and done that. Entity selection decisions are very personal because your income projections both inside and outside of the entity can influence the decision as well as legal issues. When choosing, I start with my accountant and once I have his opinion, I tell my attorney the accountant's opinion then get the attorney's opinion. This decision will also vary by state because different state's fees, etc can make a difference. For example, I don't use LLCs in California for investment entities because the fees to the state get very high when the income climbs above certain thresholds.
In CA I use limited partnerships. My investors invest in the LP and receive a split of the profits. These investors are equity investors, not debt investors. It's a pretty simple structure. The general partner of the LP is an LLC of which I am the managing member. This gives me the limited liability personally that the LP by itself won't give me. This LLC wasn't formed for liability protection however, that is what insurance is for (although the added protection of the LLC isn't lost on me), instead it was formed because I have partners and we needed a structure for the business itself. It could just as easily have been an S corp, but my CPA advised to go with LLC because he is very familiar with my income situation and thought it might have a slight edge. My attorney agreed because the LLC gives us incredible flexibility in the structure and arrangement between partners.
In Texas, I use an LLC. Now I don't have to do the LP+LLC structure that I do in CA because TX doesn't have the ultra-high state fees. Even though I don't have to, I still do anyways because all of my entities are managed by the main LLC. I guess for the TX entities I get double liability protection (triple if you count the insurance). :) Otherwise, same deal for the investors except they are LLC members not Limited Partners. Semantics.
For debt partners, they just get a note and deed of trust securing a particular property. No entity structure needed except that the entity having ownership of the asset is the borrower and trustor. If I owned the asset personally, I could have no entity at all and I would be the borrower/trustor.
Finally, you asked how do you position your business for success?? For starters, entity structures don't position you for success, they just give you a vehicle from which to conduct your operations. What positions you for success is your strategy, your market acumen, your TRACK RECORD, honesty, integrity, and results. These all take time to develop, and many newer investors fail because they don't have the patience to do the steps necessary to attain these things.
I hope the answer to your questions was in there somewhere!