Try to book non-full flights, and ask for a seat to be blocked next to you.

If you sit near the back of the plane, in an aisle seat in the middle section, it will be easier for them to accommodate this request. They normally can only block a seat once you get to the airport to check in. They may put a remark in your booking ahead of time, but you need to check they'll action it on check-in.

Also, use your points for upgrades