We prefer to buy and hold, but we did a flip last year with a partner. For clarity I/we =DH+me, She=friend+her DH. I'm aware that the following is a little confusing, but that is precisely the point.
She is a RE Broker. She found the property and made the deal. We went in 50/50 and paid cash. She waived all commissions. DH and I did all the improvements and paid for them up front. Agreement was to split cost of work/materials 50/50 at COE. As we got close to the end, she found the buyer, so no commissions to any other agents. This made the project less fun because now we had a hard deadline. We agreed to use an outside contractor to expedite the last portion of the rehab (deck, so not a problem to have another contractor on site) and She agreed to pay him directly, since She had secured a buyer before we were done with the work.
In the end, She reimbursed us for 50% of our expenses (except the deck work, which she paid for separately), and no commissions were paid. We split the profits and both walked away with about 70K, so not bad, but we worked nights and weekends for the better part of three months. We did the physical part of the labor, she did the deal making. It worked out fine, and some day, we'd consider doing another deal with "She", but not any time soon ;-)
My point is these deals involve metric crap tons of moving parts. A lot can go wrong, so knowing what you're getting into and spelling as much as possible out in advance is vital. We did our deal with someone we know and trust. We had some tense moments, but our friendship and our wallets survived intact. Hope that helps!