First off and most obvious choice for preventing flats is your first line of defense and that's the tire, plain and simple. Yes I agree that Schwalbe Marathon model line of tires are the best against flats. You didn't say what tire size you use or what you use your bike for. If you use your bike for loaded touring then the best you can get is the Marathon Plus HS440, this is as flat resistant of a tire you'll find anywhere, plus it wears like iron, it's not uncommon to hear reports of 6,000 miles on a tire, but it's not a lightweight tire at 900 or so grams, and the ride quality is a bit stiff, the rolling resistance isn't bad about in the middle of the road for similar tires. The company calls these tires flatless...I doubt that, I do use one of the rear on my touring bike and haven't had a flat but any tire can have something unexpectedly bad happen.
If you want a highly flat resistant tire that's no so heavy as the Marathon 440 and rolls faster than other tires of it's kind, it will last about 4,000 miles and probably longer on the front, which is where I put mine at, is the Marathon Almotion HS453. I haven't gotten a flat on these either but they are on the front and fronts get far fewer flats than the rear does. I know you're thinking it's an odd thing that I did by mixing up the tires on my bike but I wanted to reduce weight a bit while decreasing rolling resistance a bit as well, and they do feel better than the OEM Kenda Drumlin tires that came with my bike, those Kenda tires weighed 1,600 grams a piece so I saved quite a bit of weight doing what I did.
Beyond the tires I haven't done anything else on my touring bike, when I was using the Kenda's I did have a Panaracer FlatAway Kevlar liner in the rear only, but while it wouldn't allow me to push a tack through it when it was new it slowly wears out inside the tire which deposits yellow fuzz all over the tube, and before I replaced the tire I got a couple of flats in a roll, so when I pulled out the liner I noticed it was thin and I was able to push a tack through it easily. I have decided for the time being to go without liners, I think with the tires I'm using all it would do is add unnecessary weight and not gain anything in flat protection.
The reason I don't use liners any more in my touring bike is similar to why I don't use sealants. I don't like sealants to begin with, sure initially the liquid is lighter than a poly liner and they do work at sealing tiny holes, but after 3 or so months you have to put more in and then that liquid becomes heavier than a poly liner. Of course with a tubeless tire you don't have a choice you have to use sealants which is why I will never use tubeless tires, plus if you have a flat with a tubeless on the road you have to put a tube in anyways, just seems stupid to me, and it seems equally stupid to put in sealant all the time with that weight going up and up every time you add it to a tubed tire as well.
So I think that if you have a good flat resistant tire all the other weird stuff isn't needed. Now on skinny road tires if you ride in an area prone to have thorns then you may need a liner, in that case I would use the Mr Tuffy Lite liner, I haven't used those yet because I was using the Panarace Flataway in my rear road tire as well, but since that liner also wore out I will be buying the Mr Tuffy Lite liner for use in the rear only, then put the other liner (it comes with 2 in package) in my commuter rear tire.
There is also new poly tubes on the market promising to be more resistant to flats than any other type of tube. Problem with those is the cost hovering at around $40 each! The other problem, which I have emailed several places that make these tubes and none will respond to my question, is that you have to use a special glueless patch they make and will sell you, what they won't mention on their websites or tell me is if that patch is a permanent patch or just to get me home solution then have to throw the tube away because the patch won't take to the tube for more than 24 or so hours. I'm not throwing away a $40 tube!! So until they can promise us that the glueless patch is a permanent fix I cannot suggest that alternative for flat protection.
Sorry for the long answer but I hope it helps someone.