Twenty years is a long time for a relationship by any stretch, but it is especially impressive in this day and age considering that, according to some statistics, 41% of all marriages in America end in divorce. So a 20th anniversary is undoubtedly worthy of at least a card, one with a little sentimentality, a sense of humor to keep things light, and good wishes for the future. What to write in the card for your spouse depends on the card itself and the tone you want to create, which should reflect the nature of your relationship.
Make it sentimental...
This card is obviously a token of your love for one another, so if you're going to keep it entirely sentimental, be sure to highlight certain things. Perhaps let them know of the little things they do in your life that only you notice, that you'd miss the most if they weren't in your life. Perhaps you should reminisce on how you met and all the trials and tribulations you may have been through in your marriage--and how the two of you have survived them.
...but also make it funny...
Humor is often the sign of a healthy person and a healthy relationship. Though sentimentality should be in the card somewhere, a good dose of humor can make a card special, too. Including jokes that poke fun at traditional sentiments about marriage are always funny, but poking fun at yourself will undoubtedly give your spouse a good laugh. This is a happy time which should be full of laughter and joy--what better way to trigger those feelings then by making your better half laugh?
...and look to the future.
And last, but certainly not least, whatever is written on your 20th anniversary card should look to the future, the next ten, twenty, thirty years--eternity perhaps. This particular anniversary is just a moment in a relationship that is supposed to last a lifetime, and you should acknowledge that you are only part way through your journey together and that you have great hopes for your future.