One word answer, Umbrella, okay should be two, Umbrella Insurance. LLC, series LLC while interesting concept in Real Estate world, but unless you follow proper guidelines, you will not be able to use them to protect anything. Also with LLC you will need to get commercial insurance, and also for loans, many banks might not give loan for an LLC, if you still put your name there, you will be on the hook (searching for piercing the veil). And even when done right, they only provide inside out protection best case, and not outside in(or atleast not cheaply).
You will also hear about anonymous LLC, out of state LLC(like Wyoming, delaware), all kinds of complicated stuff, which will result in annual costs, besides all the extra work, and all this will cut into your cash flow.
Umbrella insurance, is simple, easy to get. Once you are doing big investments and especially looking to have partners, definitely go for it, until then, my $.02, you do not need one. Also try not be slum lord and do everything in writing.