I find these tips very helpful:
1. Let people around you know you are a Christian (in a natural, unforced way)
2. Ask friends about their faith - and just listen!
3. Listen to your friends problems - maybe offer to pray for them
4. Share your problems with others - testify to how your faith helps you
5. Give them a book to read
6. Share your story
7. Answer objections and questions
8. Invite them to a church event
9. Offer to read the Bible with them
10. Take them to an explore course
Hope these are helpful. Keller advises to start with 1-4 and see if people are interested and want to talk more. Then move to stages 5-7. If they continue to show interest move to the last 3 stages.
Grace upon grace,