I'll attempt to answer this using pseudo-science and reason.
The following logic applies to spells that are emitted through use of the wand and therefore travel through space/time to reach their target. It also assumes that the effects of spells are constant, regardless of range (i.e. Stupify would have the same effect if the target is 5m away as it would if they were 50m away). Finally, this answer assumes that spells are affected by muggle-world physical constraints. If any one of these assumptions is proved to be invalid then this answer, either in its entirety or in part, may also be invalid.
We can assume from the fact that many offensive spells can be dodged or blocked/countered that they are travelling at slower than the speed of light. The answer to this question also backs up this theory: How Fast do Spells Travel in the Harry Potter Universe?
Assuming that these offensive spells have a mass (which is a logical assumption because many offensive spells omit light and sound and travel slower than the speed of light), they will be affected by gravity, so there will be a drop of some sort over long ranges.
With regards to accuracy, the spell is usually emitted from the tip of the wand, held in a single hand. This is akin to firing a pistol one-handed - although you see this in gangster movies and westerns, it's actually very difficult to accurately fire a pistol one-handed over any sort of distance. Pistol practice and/or qualification often takes place at ranges of less than 25m, with the general assumption that combat would take place within 10m. This explains why so many shots miss, especially in the aerial combat in Order of the Phoenix.
In addition, the shape of the spell should be taken into consideration. Many spells shown on the film adaptation seem to have shifting shapes, which would affect air resistance and alter the trajectory.
Long-range muggle bullets use rifling to spin the bullet, therefore allowing the bullet to make better headway through the air. If a wizard could train him or herself to constrain the physical aspects of the spell as it travels to its target (make it smaller and less convulsive), this would most likely aid him or her in producing more accurate spells.
Finally, as I already mentioned, the spells emit light and sound - Firing the spell from a longer range would no doubt give your intended target more time to evade or produce a counter-spell. Seeing as many spells can already be evaded or countered within the short distances they're used at in the books and films, this might have a significant impact on the effectiveness of long-range spell-casting.
Due to the fact that these offensive spells are essentially close-range, low accuracy spells, fired from a single hand, I don't think we'll be seeing wizard snipers any time soon.
However, if a contraption could be devised to hold the wand steady (I imagine this would look similar to a muggle crossbow), and the spell could be adjusted to give it a smaller profile, less mass and emit less or no light or sound... Well, who really knows the limit of possibilities when it comes to magic? One thing I do know is that sniping would be considered bad sport, and would almost certainly be outlawed by the Ministry of Magic, in a similar fashion to the three unforgivable curses (see: What makes those three curses unforgivable?).
