Use a clear, firm, confident tone of voice.
Make direct eye contact with the other person. Stand/sit straight to create a confident air.
Do not feel guilty about the refusal and remember, you will not hurt anyone by not drinking/using.
“NO” should be the first word out of your mouth.
Suggest an alternative (e.g. something else to do/eat/drink).
Request a behaviour change so that the other person stops asking (e.g. ask the person not to offer alcohol/other drugs anymore).
Change the subject to something else to avoid getting involved in a drawn out debate about using/drinking.
Avoid using excuses and avoid vague answers, which will suggest to the person that at a later date you may accept an offer to use/drink.