The most important would be to develop and plan a regular exercise routine. Obviously there are multiple benefits to exercise, but weight bearing exercise is crucial for strengthening bones and joints. Examples of weight bearing exercises would be any form of low impact cardiovascular movement such as walking, hiking or classes such as yoga or Pilates. The resistance movements that one needs to put emphasis on are axial exercises. Examples of axial exercises are squat or lunge movements, which promote load down through the spine/pelvis/hips. The focus of the movements should be around the back and hips as this generally is the most affected area.
High intensity exercise or excessive impact should be avoided. This is movements such as high intensity cardio, like body combat or sprint training or jumping exercises such as plyometrics or power style training. The focus needs to be more on low impact cardio which can also be referred to as slow steady state training (keeping heart rate at a constant beat, not going up and down with intensity). Osteoporosis can really be a crippling condition and one should focus on staying fit and active as much as possible. There are obvious, practical situations that one who has IBD needs to plan for, for example knowing where toilets are situated before training.
If you are not getting enough calcium in your diet due to dairy intolerance, you may need a calcium supplement. Vitamin D is also essential in bone growth so discuss with your doctor about getting these on prescription. One should also restrict the level of alcohol consumption and avoid smoking as both these factors increase the risk of bone loss.