I sometimes supplement with electric wall units in the bathrooms (since people like to be toasty coming out of the shower).  

If I needed to supplement 50% of PITI for a few years or longer, I am in position to do so. 

But, in apartment syndications, you're capital will likely be tied up for 5 to 7 years, unless the syndicator sells early, refinances or secures a supplemental loan.Another option would be to find a multifamily syndicator who takes on debt investors.

@Michael Marquis Angie is right that using some technology and basic CRM automation off your own website is a good supplement to what you're doing as far as personal networking. 

We supplement personal properties with pure investments.

Beyond that, start moving up to multi-family properties with the intent of building an income stream that can supplement then replace my working income stream.

I have been working part time as an editor to supplement my wife’s income, when I find out we are expecting our fourth child.

I too am looking into getting into Real Estate investing to supplement my 9-5 and hopefully transition into it full time (easier said than done, but we have to get started somewhere).  

I am looking to supplement my income with some buy and hold properties.

We have now decided that Buy/Hold small MF properties may just be the answer we were looking for to supplement eventual military retirement income.