To add a few more tips to the above:
Be sure that your docs eventually puts you on the right dosage (2mg to 2.5mg per kg bodyweight). It's fine to start lower in the first few weeks, but eventually only in this range does imuran really work.
Related to that is that you should review your blood tests yourself, looking out for your white blood cell count. Eventually it should be in the low normal range, idealy between 4-5 (normal range is 4-11). If it's slightly bellw 4 that is also fine, but it should not be significantly below. If you are flaring when you take imurn initially the count may be all over the place and much too high... Just so you know.
Be sure that your doc orders a full blood count and liver test per blood test. It is important to monitor the liver function when taking imuran. To help your liver (that's a general thing, even for those not taking imuran) try to drink 2 liters of liquid per day, preferably water or tea rather than soft drinks. Also, excessive and permanent use of alcohol should be avoided while on imuran.