For example, if you finally promise to yourself to learn every single scale, but at the moment you barely know scales with two sharps and flats, it will take a huge amount of effort to learn them all. Split your big goal into smaller ones! Give yourself the first month to get familiar with scales with three sharps and flats, then add the next group.
If your New Year's resolution is to practice more, don't expect to increase your practice time by 3 hours every day. You may resist for a couple of days, but not more because your body and mind aren't trained to such commitment yet. Start by adding 20 minutes and later add some more, and then more again!
Don't focus on your weaknesses. Use your strengths as starting points! If you don't like your sound in the upper register, don't start from the highest note! Choose the note in the middle register you find really beautiful and then increase the range of your "good" sound.
Don't exaggerate with the number of your New Year's resolutions. If you have many of them, in every field of your life, practically you desire to become a completely different person. It's is impossible to change everything in one day. Choose maximum 3 goals and work on them! When the new habits become a part of your personality, work on the next goal!
Choose the smaller goals you can reach in a short time! We all want to see some rewards for the work done. If you have to work for half of the year to see any progress at all, it is very likely you'll get "lost" in first two months.
Write down your musical goals (I find the flute case to be a great place to store a card with written goals) and put in some place you can see them every day! We make our New Years resolutions during Holidays when we can have a rest from our work and worries. But unfortunately, Holidays don't last forever. When you have to get back to work and your mind is full of problems to resolve, students to teach, projects to make, it is very easy to forget everything else. You need a constant reminder every day for your resolution to become a habit!