General:
* Guarantee
* Secure
* Safeguard
* Assure
* Certify
* Confirm
* Verify
* Make certain
* Make sure
* See to it
* Take steps to
* Guarantee
* Underwrite
* Undertake
Formal:
* Guarantee
* Secure
* Safeguard
* Certify
* Affirm
* Validate
* Stipulate
Informal:
* Make sure
* See to it
* Double-check
* Cover your bases
Specific situations:
* For safety: protect, shield, safeguard, secure
* For success: guarantee, guarantee, make certain
* For accuracy: verify, confirm, validate, authenticate
* For compliance: comply, fulfill, meet, satisfy
The best synonym will depend on the specific context and the desired tone of your writing.