What are your suggestions on the best method for purchasing investment properties? I purchase single family homes and keep them as rental properties. Should I pay cash? Or should I take out a line of credit and use the home as collateral? Or should I take out a personal loan? I am in the process of starting an LLC but unsure as the best method to use to fund these deals going forward.
If you need to finance it, why not do it the old fashioned way by just getting a mortgage and putting some down? You could use cash to buy the homes and then re-fi your money out later. That would be another way. If you buy homes and leave them free and clear, it will really slow down your rate of purchasing properties. A personal loan is going to come at a higher interest rate and you probably won’t get too much money that way since it’s an unsecured loan, so I wouldn’t go that route. My wife and I have an LLC and we just get mortgages in the name of the LLC. We have to personally guarantee the loans. It’s worked well for us.
You should speak with an accountant and a financial planner to see what is the best strategy based on your assets and income.
Of course if you have money and good credit it is easy: you can get a conventional loan and out down a downpayment.
The real question is for people who want to invest, but have no money or credit. And yes, it is possible. I have done it myself and I teach investors every day how to do it.
Options include: Subject To, Lease Options, Joint Ventures, Private Funding, Syndication, etc.
Where there is a will there is a way! 
Grab a good realtor partner before starting your investment. Someone who knows better off in the property market and who knows more than the language you know in banks.
I have used conventional financing on my last 4 deals. I was able to get a HELOC off my first property. I have used it to fund the other three deals.
I use my HELOC to pay for the down payment, closing costs and repairs. Once the repairs are completed I refi and pay back my HELOC and then repeat.This may not be what is recommended but it has worked for me.
Personally I would not tie up my funds or take a HELOC out on my primary to funds deals. I try to keep investing as self sufficient as possible.
All depends, I prefer keeping my cash and using other peoples money and assets, and then when it is necessary to have a large amount of cash to swing a deal I can pull the trigger and get it done. Especially when interest rates are as low as they are now I’m happy to build as much low interest rate debt as I can and save my bullets.