90/180 rule

I think your dates are fine. The one-way ticket could be a problem but I've never tried it so I don't know for sure.

The issue of the one way ticket is often discussed but have not known or read about anyone actually being asked to produce a RT ticket. I think the concern is that if an immigration officer suspected that you planned to stay in the Schengen zone for more than 90 days, perhaps working under the table, he could ask to see a return reservation but in these days of electronic tickets, etc. how could you show a return ticket if you did not have your smart phone or an internet connections. But I do think I you do need an answer if he asks, "When do you plan to return?" Any hesitation with your answer may be a red flag. Why not book an open ticket? It will be more expensive but I doubt if it would be more expensive than two, one way tickets.

You had 90 days in 6 months commencing June 2014 (ending Dec 2014).
You had 90 days in 6 months commencing Jan 19 2015 (ending July 19 2015)
So you can enter again after July 19 2015 for another 90 days within the following 6 months.
They don't like you entering anywhere without a return ticket or correct visa. Check up on that one.

There is a number of sites like this if you google for them: http://www.visacalc.com/

With so much time recently in Spain, a one-way ticket may raise suspicions.

Yeah that's what I am worried about that they will find it suspicious since I have been traveling around Spain so frequently. The only reason for the one way is because I have an outbound flight from Santander, Spain to London where I am meeting friends so I assumed that would suffice as proof that I am in fact leaving the area.

The Schengen visa waiver program is for tourists and occasional visitors.

Your returning there for extended periods of time frequently may well raise issues that you are not a bona fide tourist.

I have been asked to show a departure ticket when entering Britain, but I had one, so the issue didn't go any further.

I imagine the airline is going to ask you either to show a visa or a return ticket. They don't want to be responsible for having to put you on the next flight back if the Spanish authorities refuse to let you enter.

(I'm presupposing you are a U.S. citizen departing from the U.S.)

but of course none of us here really know, since none of us are immigration authorities in Spain, nor do we work for the airline that will be examining your documents before they put you on their plane for the trip over.

Since London is not in the Schengen zone your onward ticket to London within the 90 days would, I think, be enough to show them you don't intend to overstay your 90 day visa free allowance.

Since London is not in the Schengen zone your onward ticket to London within the 90 days would, I think, be enough to show them you don't intend to overstay your 90 day visa free allowance.

This is correct. If they even ask to see a return ticket, which is likely rare. It might be more likely as you may have to tell them you are staying 90 days and they may well see you have done this a few times before. But having done it before may also make them feel you aren't violating the rules.

Be prepared to answer any questions honestly and show that you are leaving the Schengen zone within the 90 day limit. You may even need to show proof of sufficient funds to stay for 90 days if they get really suspicious. But the point of the Schengen agreement is to allow qualified people (including US citizens) to travel freely for up to 90 days (with 90 days out). It doesn't matter that you do it repeatedly. People with summer homes or family do it all the time.

It is not the airline's responsibility if the OP is denied entry because the immigration people feel he may overstay his welcome.

If the OP is flying into a Schengen country, and he has a U.S. passport, that is all the airline needs to see. It is a visa waiver country so no visa is needed. How long you stay is not the business of the airline. For all they know, you could be flying back on a different airline. It's done all the time.

Now, if you fly into a country that requries visas, then the airline would have to check both your passport and your visa.

When asking for a return ticket, that is a technical term. What they want to see is a tickret out of the Schengen zone. You could be traveling around the world so therefore you might have numerous flights before heading "home." They just want to make sure you plan to leave.

Just an observation, in today's world of electronic monitoring, your actions or evidence at entry are less of a concern than your history of entries and exits. If it appears that you are abusing a tourist visa, you may be called out on it, regardless of "rules". The UK maybe does a better job of it, though they do not guarantee any entry. In your case I would be very prepared to explain your plans as well as explain why you have been there repeatedly on a tourist visa. They may buy it or they may not.

I really do think that everyone is "over thinking" this question. As long as you are legal within the 90/180 rule and have been in the past, I don't think you have anything to be concerned about. O'sure, we can dream up situations that might develop but are highly unlikely. Just make sure you entry and exit stamps are clear. Just last week in France I was hung up at the exit check because the officer could not quickly find my entry stamp. Easily found an entry stamp in 2013. That may have caused him to look more closely at my passport. If you duck into London for a few days make sure those stamps are there - in and out.

Thanks for the input everyone, I appreciate it very much. I will see how everything goes.
Thanks again