The Engineer's Joint Contract Documents Committee has some standard contracts for engineers. Go to NSPE website and link from there.....or you could pirate one from another engineer...that's what everyone else does.
Make sure it has some basics covered, no matter which method you use...
1. Standard of care
2. Limitation of liability (and mutual or consequential damages)
3. Indemnification
4. Ownership of documents
5. Sample disposal and management (if you take samples such as for testing or forensic work)
6. Site operations (don't assume any responsibility for such)
7. Site safety (don't assume responsibility)
8. Dispute resolution (stay away from arbitration)
9. Clause severability