Yep, holding onto a gym that is slightly more difficult to get to or so is key. But I like that they don't hold that long, makes it good since means other team's gyms are easier to take over.
There is a Church sign on my way to work that I mentioned before (one that says they welcome Pokemon GO players) that is a gym. It's easy to get to, but you kinda need to park in a nearby neighborhood and walk over to the sign next to traffic in the heat. It's been at a 9+ Gym level for weeks now. Luckily for my team Blue. Last Friday I boosted it up to lv 10, which took me like 30 minutes... lol. But got my Pokemon in it and enjoyed constant source of coins from that 1 gym for last 8 days now.
Only downside is this was before my more recent wave of top Pokemon. So only my 974 Starmie was used for it. So I'm now in the bottom 2 of that gym. So eventually... I will be knocked out as it occasionally goes up and down. But, at least if I ever get knocked out, I can prob boost it up again and put a much strong pokemon in.