In spite of the fact that they are a integral part of many cruise excursions, I hate tour buses.  They are often cramped, crowded and hot - and there is grubby glass between the locale and me and my camera. The lack of space means that it's difficult just to extract my camera from its case let alone swing it around to take a photo through the bus window.  Often the bus blows by interesting landmarks with only a brief acknowledgement of what we are seeing let alone treat us an actual stop. 

To improve you chances of getting a good photo from a tour bus, try one or more of the following techniques:

1. Use a digital derringer: with very little room to move, a compact camera is easy to remove from a case and hold up to the window to use.
2. Use a small lens on your digital SLR: if you only have your big black camera, try to use a short lens so that you don't bang your gear against the window glass. A small, inexpensive prime lens like a 50mm f/1.8 will serve you well in these situations.
   
3. Use a flexible rubber lens hood: this will reduce or remove any reflections that you usually get when shooting through glass by placing the hood directly on the bus window.  The above photo of Catedral Metropolitan in Panama City, Panama was taken through a bus window.  I had no lens hood but was lucky enough to get this shot with only some reflection - can you see it?  With a flexible hood, you can shoot from an angle and still have the hood completely touching the glass and preventing a reflection
   
4. Use a rigid circular lens hood: while losing the flexibility, even a rigid lens hood is better than none at all.  On my Leica X1 I use a black metal lens hood from a 90mm f/2.5 M lens that sticks out beyond the front of the lens.  I can then press the hood directly again the glass window and eliminate any reflection.  
   
5. Use a high shuttle speed: with scenery flying by the bus, you are going to want to freeze those moving trees, so pick a speed above 1/500 if possible or, if your prefer something more automatic, select an action or sport setting.
6. Turn off your flash: "blow back" off of your flash onto a close piece of glass is not a good thing.  You will get a huge amount of glare and will washout the image you were trying to photograph.
   
7. Turn up your ISO: some tour bus glass can be tinted - really tinted.  You may find that there isn't enough light to get a decent photo using the recommended high shutter speed.  If you camera isn't set to auto ISO, you may need to run up your ISO to 400 or more.
8. Focus manually:  the auto focus on some cameras don't like having glass too close to them.  You may need to use manual focus (often set to infinity) so that your camera doesn't "hunt" for focus.
9. Sit a little further back on the bus: you can often see the "best bits" approaching the window if you are a bit further back.  That way you won't be surprised as the scenery flies by!  Pay attention to the side pillars of the bus that separate the windows - some seats will place you right  beside these making it difficult to shoot around.
   
10. Eschew to bus all together:  the simplest solution is not to get on the bus!  Look for walking tours that are often more photographer friendly due to their slower pace.  You can also look for tours where the bus is only a means of transportation to the real sightseeing rather than being the tour itself.
    

While not all lenses work well with generic rubber lens hoods.  The 50mm lens in the bottom right is perfect to pair with a hood.  Zoom lenses, like the ones in the back row,  can be fitted with hoods, but make sure you don't get any vignetting at their widest settings.  Other lenses like the fish eye in the lower left don't accept any kind of lens hood - and have a front element that sticks out - so avoid placing it up against any glass!