First pick the site. Most IM shots are now given in the neck and not the hip. Insert the needle quickly and pull the plunger back just a little, if the syringe starts to fill with blood you've inserted it in a vein and need to remove it and insert it into a different site, if no blood go ahead and apply firm pressure to inject medicine. How easy it is to push will depend on the consistency of the drug you are pushing, some are easy some are not. Would advise having the cow or calve fairly secure to avoid them from moving around much and taking a chance on breaking the needle off. Also make sure you have a new needle as it will make it easier on you and the cattle. Last thing I can think of is do not administer more than 10cc's into one site.
Also keep in mind that some cattle are extemely sensitive to any injections, either IM or subQ. and may develop absesses afterwards, even with the best of sanitary proticals. They may get as big or even bigger then a fist, and may require draining.. But keep in mind that this is rare, but I do have one cow that always gets them after injections.
Something to keep in mind when giving injections is the guage and length of the needle, as some drugs/vaccines are thicker than others and require a larger bore needle. Also if giving a cow a shot versus a calf you may need a longer needle length to inject the dosage into the animal properly.
Dull and dirty needles can cause infection, so if you're using the same needle for multiple animals, change it frequently. They aren't that expensive.
Personally I prefer the all steel needles. I've found that the "cheapy" plastic base ones bend or snap off at the base occasionally and can be frustrating to deal with.
Remember that if all possible, give it in the triangle of the neck. Then at least if it abcessess (sp?), you're not ruining a more valuable cut ie. the hip.