A lot of lifters experience lower back pumps when using steroids but the lower back pumps shouldn't be horrible if your dosage is sensible.
Here are my 4 main tips to prevent muscle cramps/lower back pumps:
1. Make sure you are drinking enough water through out the day. 5-6L per day minimum. Remember that stimulants (coffee) and alcohol are diuretics and don't count towards your total.
2. Make sure you are taking in enough electrolytes (think sodium, potassium, magnesium and calcium)
3. Supplement with 2-5g of taurine per day. Ideally you would put it in the water you drink during your workouts and sip on it through out your workout
4. Often neglected and over looked, make sure you are stretching after each workout and try to incorporate foam rolling a few times per week into your program to help remove lactic acid from tight muscles.