Remove the words "like" and "dislike" from your critical vocabulary, whether you're looking at your own art, the art of a peer, or a work by an established master. Start by looking at how it's made as far as medium and technique, then think about how the art moves you, where your eye is drawn and what it conveys to you.
Consider what makes a work of art interesting or not interesting. Analyze why you react to it the way you do. This style of critique can help you with your own art, especially if you feel uninspired.
Ask questions to other artists. In a formal critique, you will have the opportunity to ask questions about other artists' pieces. You will also have the opportunity to ask others questions about your own work -- how does your work make other people feel, and is it in line with what you're trying to convey?
Listen to criticism, even if it's less than positive. You don't have to take any advice offered, but if you get upset and shut out every bit of criticism, your art won't flourish. Often, you may be surprised when you try applying what you hear in a critique to your work. And, even if it doesn't work for you, you've experimented and grown as an artist.