Relax. It's hard to cultivate a sense of gratitude when you're angry, frustrated, or anxious. If these are issues that you struggle with, it's important to resolve them, as they're formidable barriers to thankfulness.
Live in the Moment. If you're too busy dwelling on the past or thinking about the future, you won't be able to fully notice how fantastic things are right now. Plus, thinking about the past and future opens the door to comparison, which is the only way you can perceive something as not good enough. What you have now is all that exists, and comparing that to something that doesn't exist anymore (or yet) is an easy way to foster dissatisfaction and torture yourself. Like the old saying goes "Past is History, Future is a Mystery and Today is Gift and hence it is called the Present". Enjoy today, this moment and don't postpone your enjoyment.
Start with your senses. The most basic pleasures in life are usually accessible to us all the time, but they slip out of our consciousness because we get so used to them. Learn to notice the little things, and deliberately appreciate them.
Cherish any kind of lightheartedness in your life. Things like laughter, affection, and playfulness are fleeting. Once a relationship has degraded so that those things don't spontaneously occur anymore, it's very hard to get them back. You might know that from experience. So treat those moments with care (especially with kids, who are at the peak of lightheartedness). Don't be the person who takes life too seriously, who doesn't have time to have fun, or who has no sense of humor.
Take a vacation. There's some truth to the saying "absence makes the heart grow fonder". Ideally, you shouldn't have to separate yourself from something (or someone) in order to appreciate it (that can lead to a vicious cycle). But it can be a good way to trigger your sense of gratitude just once in a while.
Keep a gratitude journal. Challenge yourself to write down five new things every day that you're grateful for. It'll be easy in the beginning, but soon you'll discover that you have to increase your awareness to keep on.
Hope you'll follow these tips!