I was detained briefly at Heathrow when I was dating my now-husband. I had flown in and out several times within a few months and was returning to England for a 2 month stay. All of the travel was within 6 months of my initial entry, so it was within the immigration rules, but they were concerned about the amount of time I was spending in the UK.
I showed my return ticket to the US. I wasn't working at the time, but told them I had a house in the US and paid over $400 per month for US health insurance.
The officer ushered me to an adjacent seating area. She returned about 10 minutes later to say I was free to enter, but I needed to be careful because if I continued to come for long stays it would look like I was spending more time in the UK than the US.
ksand24 gave great advice, as usual. Have documentation with you that you have a life in the US, you intend to return to it, and you can support yourself while you're in the UK.