What were some vital saving habits you used or still use in order to be able to invest ?

Take the money from your BAH that you won't be using towards housing expenses, save it, and then buy more property with it.

However, there are many other items that can fall under CapEx and you should save for those possibilities.  

Ultimately, it's up to your lender. 

It can allow you save money a lot faster when you eliminate a lot of your housing expense. 

On top of @Eamonn McElroy ‘s great input, I would like to add that if you need to sell the developed property rather than holding it, you might save some taxes if you sell the lot to the controlled entity before actually developing it. 

I bought the building in June and will be looking to do a cash out refinance as soon as I can  to extract the ~45k I put into it and dump it into another project.I got a MassSave (energy savings program in Mass) audit done, and they quoted around ~10k for insulation.

Or, if you would save more money paying down the mortgage than you would get in returns on the AirBnB, go that route.

If you have a $100,000 loan at 3.75% and that goes up to 5.25% but your able to get another 50-100k out the additional cost on your current loan is only $1500 per year which is a small price to pay to go create some other opportunity that over the life of that second investment can yield you $50,000-$150,000.My biz partner told me years back, "Don't step over quarters to pick up pennies", which to me most of the time translates to calculate the money you are not making by taking that next step vs the money you are trying to save.

Much better to refinance with a safe equity cushion and save up for the next investment property.