1. Incorporate.
2. Sufficiently capitalize your company.
3. Follow corporate formalities.
4. Don't commingle your personal and corporate funds.
5. Apply for a mileage-earning corporate credit card. Get it.
6. Spend to your heart's content, so long as you follow rules 2-4, above.
7. Presto, you now have a mileage-earning corporate credit card with no personal liability.
(Of course, this post is not intended to provide legal advice to the OP or anyone else. YMMV.)