Hi Shaun,
I have been using canon gear for quite a while and will probably just repeat many of the things already mentioned, but here goes.
I can't speak for the 18-55mm lens as i have never owned or used one but offer the following thoughts:
(a) L series glass is the way to go with Canon, if your budget can afford it!
(b) Be aware that EF-S lenses won't work on a camera with a full frame sensor ... so I would generally stick with EF only lenses, as you might want to upgrade one day to a 5D or the 1ds mkIII, however this comes at the cost of approx 1.6 times focal length.
(c) only buy top grade filters Uv or other.... (High End Hoya) is a good place to start - it is pointless buying a fantastic piece of glass and having a cheap filter.
(d) when you use a tripod, turn any image stab. features off, IS etc., as this only upsets things & use a cable release. (I generally forget to do this until, I have hammered off a heap of shots
)
(e) Prime lenses will give the best quality e.g 50mm but the least flexibility so depending on what you shoot, I would consider this type of lens very carefully, unless you like changing lenses a lot.
As with regards to your softness in the photos, I would start by looking into this 72dpi business, as while i don't know the camera this is obviously a setting gone a stray! if this is what you are actually getting.
You mention that you have a 75-300mm are these photos soft ???? as you move through the F stops ?
Also on the glass front i can recommend the 28-135 IS USM, until recently this was my primary lens.
Hope this helps,
Cheers,
.adrian