One way is to get a spirit leveller, but hard to level handheld. Some cameras have in built levelers.
Else sometimes I lazy I will align with verticals of buildings or trees etc. Land horizon may not be levelled if there is slope.
Horizon from a water body can be used as a leveller as well
Do adjust post processing ( ie small rotational ajustments if necessary )
If your viewfinder has a grid, use the horizontal to line up with a horizontal element in the image.
If you do not have a viewfinder grid, the other way is to use a camera mounted spirit level - but you need to use a tripod to freeze the position.
If you cannot do the above, you will have to do this by trial and error.
Take a couple of shots and see if you tend to tilt your camera to one side. Consciously make adjustment in the opposite direction whenever you take your photos. This is not a foolproof way. Do this over and over again and hope that it will be second nature.
The last way is of course to adjust the tilt when you do post processing.
some cameras have built-in virtual horizons, and perhaps if you have let's say an iPhone, you can download apps to help you. simple apps like Carpenter, or somewhere near that, have the system you need. just place them flat under your DSLR body and freeze the settings.
one additional tip though. tripods are usually vital for landscape photography due to the usually high f/stop required for the DoF
A spirit level is the best way to ensure that it is level. That said, you can have the horizontal level down pat but your horizon might not look straight. This is owning to curvature introduced in the photograph due to barrel distortion from lenses. PT Lens or Photoshop CS 5 would be able to correct this part, but the horizon would still look wonky owning to vertical perspective distortion, etc.
To add on to the list of problems with getting the horizon straight, the opposite bank in Singapore is almost always very near, even if you are at the beach. Definitely when you are at one of our reservoirs. If you do not align your camera back parallel to the bank (which is close enough, and usually there's more than one, so you would not be able to do it for all), there will be horizontal perspective distortion. Your camera might not be tilted, but because the bank is coming towards you or away from you, it would appear to be slanted.
My suggestion is to make sure that you are well aware of all these things that can make the horizon look wonky, and the steps taken to prevent them, and if they can't be helped, the steps taken to correct them in post processing.
Also, just to highlight, if you always 24-7 just make your camera level, composition will be limited. You will end up with horizon cutting through the centre or near-centre of photo. So while it is indeed the fastest and most straight-forward way to achieve it, knowing what to do after you accept that you can't use that method is just as important, if not more important.