You don't say whether you're using manual or autofocus - if the latter, there could be a mechanical/electrical fault, get it checked.
If this is a manual focus, here are some things to note:
1) the tendency is to rack the barrell round to "infinity" when shooting distant objects. In fact, many long lenses don't actually focus at infinity - they focus beyond infinity - this is a design feature to allow for thermal expansion of various lens components. The focus point (esp. at wide apertures) is very critical with long lenses - the smallest movement of the focus ring makes a huge difference. As part of your technique, try turning the focus ring either side of the correct position quite rapidly - at the correct focus point the image should appear to "snap" into focus.
2) shutter speed - while the rule of thumb says "use the shutter speed closest to the focal length" this is just a guide - if you're cold or tired, if its windy, if the lens is heavy or doesn't balance well can all significantly affect your ability to steady the lens. You will need to eliminate camera shake as the possible cause - take some tests shots with the camera securely mounted on a tripod.
3) the lens - the sad fact is that the design of (most) zooms is such that their poorest performance will typically occur at maximum zoom. You don't say which make yours is, but I've learned the hard way that cheap 3rd party zooms are a false economy. However, the most serious problems are normally only apparent when the lens is wide open - if you can stop down to, say, f8, you might find a significant improvement.
Ok - so you've got to maybe shoot at a faster shutter speed AND stop down as well - probably means faster film ... but a bit of grain is better than out-of focus!
Colin K. Work, Pixstel