A good lobby, or pressure group should assist the process of government, either by offering expert assistance, or by involvement in constructive protest.

Identifying the issues.
What do you want to see the MP for anyway?

Researching the subject involved.
Look at the issue.
What's the problem?
Why?
What are the arguments for your position.
Why should the MP care?

Letter writing.
Not too detailed.
Outline the problem in a few sentences.
Ask for the meeting.

Local PR
Send aPress Release. 'MAG Grounchbucket has a problem with X and will be lobbying the local MP'

Setting up meetings
What was the MP reply like? Hostile/non-committal/Good?
Oiling the wheels. Getting to know his or her staff

How to prepare
More research.
Find out more about the MP -- likes/dislikes
Contact Region or Central if you need advice or briefing notes.

Who goes?
Not necessarily rep. Best people for job.

Formality:

During the meeting.
Introductions
Presentation of argument,
Break it down so that one person leads, other 'cleans up'.
Two sections, each dealt with by a different person.
Finish by asking MP what you want him/her to do for you
Allow time for Questions.

After meeting.
Developing the relationship
Small talk helps unless they are very busy.
Photographs?
Send follow-up 'thank you' letter, outlining what you saw as the main points of meeting.
Depending on circumstance, tell press.
Tell MAG Central about it.