Before asking for help, it is important that you are dressed appropriately. Wearing formal business attire will be the best way to land a business class seat. It is important that you keep yourself busy, where you are constantly fidgeting with your cell phone, or carrying a laptop etc. People will notice you and may just be inclined to hear you out.
Ask whether there are empty seats available. The best way to do is to ask politely. However, make sure that you do it in time, preferably close to the take off time. Although this could backfire, requesting too early will not work in most cases where airline attendants are under strict orders. Make sure that you choose the right time by evaluating the scenario there and then.
Look for an alternate flight. You can do this on your own by searching the net which will save time. However, make sure that an alternate flight is available if you are considering the option of rescheduling it. Most often than not, the airline will ask for your flight confirmation number and may be willing to upgrade for free. Contact the airline by phone as online booking may result in a small fee being charged.
If you are a frequent traveller, you may be qualified for an upgrade, subject to a valid certificate. This could be helpful when flights are oversold.
In most cases, you can volunteer to give up your seat. This strategy can be extremely helpful as airline attendants may be willing to upgrade you on the next alternate flight.