I'm looking for recommendations on how to best use miles for an infrequent flier. I take an international trip every couple of years and typically go skiing about once per year. I currently have around 17,500 points. What is the best way to make use of the points? Upgrades for the short hops when skiing? Save up to pay for the flight on a ski trip? Save up for an International trip? Upgrade on an international flight?

Sadly, I have never been able to make use of points/miles so some of the features are a bit fuzzy. I don't quite understand upgrades. Common sense would say you upgrade to a higher class. The United website shows US upgrades are 5k for economy and 15k for business. Is the 5k to economy plus? business? first class? Why is the business upgrade more? As for using the points to secure a free flight, are the seats available for rewards so limited that International is difficult without lots of flexibility? How about stateside flights to ski resorts during the winter?

Any input is appreciated. I would like to take advantage of the miles in the best way possible.