There's no point in wrecking your head over this. Just a congratulations and a hug with the lovely couple will do fine! If you are not in person, arrange for a small gift or a bouquet of flowers to be given to the couple, and add a greeting card with it. Greeting cards a just great because you don't have to think of what to say - it is usually already written in the card. All that's required from you is to sign the card.
Getting a gift is a good idea. If you are very close to your soon-to-be wedded friends, it would be a great gift if you could help them out in the wedding planning. If you are the bride's friend, accompany her to the dress fitting, cake tasting or food tasting sessions. If you are the groom's pal get him fitted for a suit. A bachelor or bachelorette party for the engaged couple might be fun as well.
Look online if you can`t think of the right words. Or perhaps some words from a favourite book of poetry. It is always better to write too little rather than too much as it can easily sound awkward. ``Congratulations´´ is always acceptable. ``Wishing you a lifetime of love and happiness´´ is also a lovely sentiment. ``Wishing you a long and happy marriage´´ is also a good idea. Be careful if you try to be funny - it can easily flop and sound stupid.
So your friends are getting married and you want to give them a positive and meaningful message on this very special and significant occasion. But every time you try to put some words together they just sound awkward! Well, how about choosing a lovely wedding poem (which you could find on the internet or in a wedding book) or perhaps their favorite Bible verse or the words of a song which you know they both like. Then you could get these words made into an attractive framed poster or embroidered as a wall hanging for their new home. In this way your wedding wishes will be a blessing to your friends which they can keep and enjoy over and over in the coming months and years.