The first thing to try is actually being good at your job, the respect pretty much goes along with that. I moved into an internal IT role after several years in a field engineer position so I have been lucky enough to experience lots of different environments and deal with lots of different industries. I have worked in some places that I would never go back to and I have dealt with some people I could happily punch but now I have found a pretty sweet gig and I'm pretty set to stick about.
If the users are being rude and aggressive towards you then either you need to be able to have a quiet word to senior management about this or you're best off out of there. If the company has no respect for IT then it could just be a product of the industry you are in (a lot of manufacturing type companies see no value in IT and so don't give it any resources, compare that with industries like transport and finance who are more likely to have someone very senior responsible for IT).
Who's your line manager? Who do you report to? Surely it would be an idea to have a word with that person and try to gain some support for what you do amongst senior management. They can then come down on anyone who treats you like $h!t. The only time I ever had someone be particularly rude to me was a lass in the training department who was a little bit full of herself. I had a word with the IT director who then had a word with the HR director who then had a quiet word with the girl in question who bucked her ideas up pretty quickly and was never rude to me again. If it's a small company and you are reporting directly to the MD then you could have a chat with them and ask if they could have a chat with department heads or just generally jump on anyone who gives you attitude, companies have an interest in making sure that people like you are kept sane!
And if all else fails just mark out your problem users and work to rule with them. They'll pretty soon see that a smile and a please gets them a lot further than shouting and screaming and jumping up and down (just make sure you are firm but fair and do the bare minimum for them to satisfy your responsibilities but be warm and helpful to those who make the effort to be polite to you).