Getting a free upgrade to business class on Singapore Airlines (or any airline, for that matter) is typically not something that can be guaranteed or easily obtained, as it depends on various factors such as availability, your frequent flyer status, the airline's policies, and sometimes luck. However, here are some tips that may increase your chances of getting a complimentary upgrade:

1. Frequent Flyer Status: If you're a member of Singapore Airlines frequent flyer program (KrisFlyer) or a member of a Star Alliance frequent flyer program, such as United MileagePlus or Lufthansa Miles & More, your chances of receiving an upgrade may be higher if you have elite status. Airlines often prioritize upgrades for their loyal frequent flyers.

2. Bid for an Upgrade: Some airlines, including Singapore Airlines, offer passengers the opportunity to bid for an upgrade to a higher class of service. Keep an eye out for emails or notifications from the airline inviting you to bid for an upgrade closer to your departure date.

3. Check-In Early: Occasionally, airlines may offer complimentary upgrades at the check-in counter if there are unsold seats in the premium cabin. Arriving early for your flight and politely asking if any upgrades are available could potentially result in a free upgrade if there are seats available.

4. Special Occasions or Milestones: If you're celebrating a special occasion such as a honeymoon, anniversary, or birthday, it doesn't hurt to mention it politely to the airline staff during check-in or at the gate. While not guaranteed, some airlines may occasionally offer complimentary upgrades as a gesture of goodwill.

5. Flexibility: Being flexible with your travel plans, such as volunteering to take a later flight if the flight is overbooked or being open to alternative routing, may increase your chances of being considered for a complimentary upgrade if it helps the airline accommodate other passengers.

6. Complaint Resolution: If you encounter any issues or inconveniences during your journey, such as a problem with your seat or in-flight service, addressing them politely with the airline's staff may sometimes result in a gesture of goodwill, such as a complimentary upgrade on your next flight.

Remember that free upgrades are not guaranteed, and airlines have specific policies and procedures for determining who receives them. It's always best to maintain a polite and respectful demeanor when interacting with airline staff and to manage your expectations accordingly.