Long-term Cash Management Strategy

[Account Closed]

I am new to real estate investing, but a big issue I see in terms of long-term profit is this:

When you have bought a property and begin saving up for the next property, what do you do with the cash you are accumulating in the meantime?

So let's assume you plan to put $30,000 down on each property you buy, and you would like any additional money you accumulate to go toward new investments as quickly as possible rather than paying down the first house.  Here are the strategies I can think of:

1) Save up $30,000, buy a house, then simply save until you have $30,000 again (with the money in a savings account or stocks depending on risk appetite), then buy again, rinse, repeat, etc..

2) Save up $30,000, buy a house on a 15-year fixed mortgage to get the lowest possible rate, then when you have enough combined cash and cashable equity in your property, immediately do a cash-out refi and buy another house.

3) Save up $30,000, buy a house, then when you have enough combined cash and cashable equity in your property, take out a HELOC and buy another house. Then aggressively pay down the HELOC to avoid as much interest as possible, but as soon as you have enough cash + equity built up between your two homes, go ahead and take that money right back out of the HELOC and buy another house.

Obviously there are some important variables here, like the interest rate on your mortgage and the interest rate available when you are ready to do a cash-out refi or take out a HELOC. I am wondering if there are any general rules of thumb here though.