A pharmacist at any new job is going to have a higher error rate than a pharmacist who's been working that job for awhile. This is going to be even more so for a brand new pharmacist (or intern) or someone switching practice sites (ie retail to hospital or hospital to retail.) Every job has its unique variable--learning the computer system, learning the workflow, learing the doctors handwriting & unique prescribing habits, etc.
Experience and practice will lessen your error rate. But all humans make mistakes, and while people don't like to admit this, all pharmacists make mistakes. Even a 0.01% error rate leads to a lot of mistakes. Take the time to check thoroughly, don't let your boss or a customer make you rush....better to get fired for being too slow, than to be facing legal problems because of a misfil.
Learn about common errors & then be extra diligent in checking those medications. Always triple check high risks prescriptions (ie coumadin)--you are going to make mistakes, but you don't want it to be on a medication where a small mistake can have disastrous results. Regular release vs SR release meds are a common source of errors, so carefully check the dosage forms for medications that are available both as IR & SR. Amoxicillin & Augmentin are also common mixed up, so check those carefully.
ISMP has a newsletter that highlights errors--it's a good source to learn from the errors or potential errors that others have made. Knowing the potential errors that can be made, will help you to be aware of them & to check for them.
http://www.ismp.org/newsletters/default.asp
It's best to read the script yourself, THEN check what the technician typed....otherwise confirmation bias can lead you to not catch mistakes.
Think about the routine that works best for you. It may take some practice to get your routine right & will depend on your your computer system is set up, so that you are always checking the elements of the RX in the same order. Skipping around the elements, makes it likely that something won't get checked, that is why its important to check the elements in the same order on each RX.
I would expect that most preceptors will be understanding of people making errors during rotation--rotation is supposed to be a learning experience. Just make sure you learn from your error, think about why that error may have happened & if there is anything you can do to prevent the error in the future. As others have mentioned, make sure your blood sugar is stable--eat snacks & drink water as you need to, to keep your concentration focused.
You will undoubtedly improve as you gain more experience, so hang in there, and try not to be too discouraged by errors.