Bar chords are notorious for being difficult to play. They involve all of your fingers, and your index finger has the hardest job of all. It has to hold down multiple strings. Obviously bar chords are difficult to play, but there are a few tips for playing them.
The first tip is to flatten your index finger where you want the bar. Next get the other fingers in position for whatever chord you are forming.
Now here's the tip: roll your elbow in towards your side. This will cause your finger to roll towards the neck of the guitar and will give you some good leverage to help hold down the strings. Focus on putting pressure on the strings that your pointer finger needs to hold down. If you are playing a major chord position, then your pointer only needs to worry about a few strings. It may even help to arch it a bit. If you are playing a minor chord shape, then use your middle finger to help add weight to the index finger. Notice also that you will actually be using the side of your pointer finger to hold down the strings.
The second more obvious tip is to build your hand strength and finger strength. You can do this a number of ways, but the best is to stick with guitar and keep trying. You need to make sure and give yourself a lot of time with bar chords. They are tricky. But, when you get it, you get it!
Bar chords will open up a world of different genres of music for you including blues and jazz. They will also lay the foundation for learning unique types of chords that are very useful in song writing. For more on bar chords and unique chords, visit the Guitarmann Advanced Essentials series.