My field is facility support (utilities, mechanical, structural). My background is facility engineering. I also have training in water systems (WFI, RO, DI), so I also do a bunch of small stuff with the state regarding small non-community water systems. I also cut my teeth doing construction estimates, so I do estimating for a handful of contractors. I have a BSME, MSME. I've been a PE for 9 years. I feel like a traveling family physician. I also try to avoid engagements that are more than 200 hours or $20k. I'd rather have 4 or 5 $2500 engagements a month. Those big jobs tend to lead you down the path of porpoising (work, no work, hunt for work, work, no work, hunt for work), because you're too busy doing the big jobs to pay attention to maintaining contacts and a book of business.
Very enjoyable, doing this on your own, but 2009 was also very frightening. I've never seen construction drop off a cliff like that.
The big thing in making this work is getting a large enough network of contacts to keep the phone ringing. But it's not like you're bidding work. When you're a PE and you get enough contacts, they trust you and just call you regularly and support your practice. So network and make contacts.
Example: I answered an ad on Craigslist looking for help from a PE to do water system calcs. That was right up my alley. That one ad lead to steady work each month, plus regular meetings with well drillers and treatment vendors (who in turn call me now), which lead to face to face meetings with DEP bigwigs, which lead to more contacts and more work, etc.
Also, don't ever be afraid to turn down the people that try to haggle with you. I gave one builder my rates, and he said he knew a PE that would do the work for half what I charge. I told him thanks for the cup of coffee and good luck, call me if you get stuck. Also don't scrape the bottom of the barrel looking for work. I did that early on, and booked work doing CAD drawings for $40 an hour for home remodelers. All that did was screw with my focus. I stopped, and now I keep my focus on calcs and assessments on commercial and high end construction. More enjoyable work when you're on the roof of a casino, or working in an NFL team owner's mansion, and not in the middle of some guy's do-it-yourself basement remodel sizing up columns. Not that I turn my nose up. It's just more enjoyable.