Question: I have purchased my first DSLR (a Canon 500D) and have been playing around quite a lot within the aperture priority mode, with auto shutter speed and ISO.

Anyways as I would like to become a better photographer I would like to start taking *all* my photos in Full Manual mode.

Are there any general rules that I should use to help me select a shutter speed and ISO combination? Currently I am using trial and error and while I get there in the end, it often takes a while. I understand that I will learn this with experience, but anything to get me started will help.

Further to this, do most 'Professional' photographers set these manually? The auto settings work so well for an average shot, and it seems too time consuming to adjust these for every angle and light variation.

Answer: 
First, stay away from the "icons" on your 500D. These are the automated settings, and offer no freedom to choose your aperture and shutter speed.

Once you do that, here are four simple rules I follow:

Rule #1: stay above 1/60.
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If you are using a tripod, you can go below this speed, but when hand holding the camera, always stay above this speed. If you do not, you will tend to get blurring from camera movement. Yes IS can help....but just remember the rule, its easy.

Rule #2: Keep your shutter speed 'above' your lens focal length.
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This also helps ensure reduction of blur from camera shake. If you are shooting at 200mm, be sure your shutter speed is above 1/200th a second. If you are shooting 75mm, stay above 1/75th. If you are shooting 35mm, stay above...careful...1/60th.

Rule #3. Remember to choose bigger aperture numbers when you are shooting things farther away.
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When shooting landscapes or other scenes, or when you want your background in focus, choose a "bigger number" for aperture. This actually selects a smaller aperture, but the result is that you get more distant objects in focus, even if you are focusing on a subject fairly close by. Basically, the camera is 'squinting its eyes" , just like you do, trying to bring distant objects in focus. This suggests that you can use aperture to manage the 'look' of your image, which indeed is what you do: pick an appropriate shutter speed (rules 1 and 2) and then adjust aperture until you get the look you want, within rules 1 and 2.

Rule #4: if you can not take your shot within any of the above 3 rules, adjust your ISO up, until you get a rule to apply.
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So if you shoot at ISO 100 or 200 (and you should basically all the time), move up to ISO 400, then 800, etc until you can follow the rules.

Finally,

Rule #0: Check your ISO DAILY!
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Always check your ISO before you use your camera, as you will be very upset when you shoot a critical event at ISO 3200 and have awful grainy shots when you didn't need ISO 3200.
In general you want to use as low an ISO setting as you can to avoid noise. If the lowest ISO means shooting at a low shutter speed, say 1/15th then you may bump up the ISO a bit so that you can shoot at a faster shutter speed. 

If the light isn't changing, setting everything manually means that you get consistent exposures. Think of a sports or events photographer. If you leave in Aperture priority, then as your background changes (more sky in the photo for example) the camera may adjust the exposure and you end up with under or overexposed images.

Nothing wrong with using Aperture or Shutter priority if it suits you. Full manual yields more consistent results when you have time (and remember you're in manual!) and especially when you'll do a lot of shots in the same light.