I've been asked to write a recommendation letter for somebody's application to the religious life... any tips?

As a pre-writing activity, ask the applicant what he or she wants it to say. Then look at what needs to be said (if available). Then use this site to put it together.

This is just one site that has useful information.

Write Express: How to write the perfect reference letter.

If time allows, write it, set it aside for a day or two. Then re-read it, checking for all those little goofs that inevitably show up. If permissable, suggest some one else have a look as well.

If you're comfortable, suggest the requestor read it to make sure it says what the applicant wants.

Make it your best piece of writing ever; it will reflect well upon both of you.

I've written letters of recommendation. The tips above are very good and will help you immensely.
Another tip I have is to be specific if possible ie- rather than saying "I've know X a long time," say "I have worked with/whatever X for 2 years, and in those 2 years I have observed...."
Also, try to quantify/materialize some of the qualities you describe. Rather than "X is kind, humble, compassionate, and cheerful" work with "X's compassion toward those X serves is always evident. An example of this is.... (or) X's love for the Lord is manifested in X's constant joy and cheerfulness. (and then give an example of when you have observed this)
Also, do some research. Look at the order and their charism to see if there is some way you can connect the way of life that person will be living with the way they are living now. (IE- if a young woman was applying to the Sisters of Life, I would be sure to mention her pro-life activities).
Most importantly, pray for that person in a special way. Make the letter you write your prayer for them. For some of the letters I've had to write, I've drafted them in adoration, before the Lord, asking the Holy Spirit to guide me. Praise God, and realize that you have been asked to take part in a very special way in this person's vocation, so ask in prayer if there is any other way He can use you as well.

In Jesus and Mary,
Lauren

What they said is pretty much the perfect advice to writing the letter! Good Luck!!

Whatever you do, just don't say they are interesting. That seems to get misconstrued. Also remember when writing a reference letter, for anything, that if you say something not true, or don't mention something that you know would be important, you can be sued. That happens when old bosses give good references to someone that was fired for embezzlement, and then embezzle at their new employers. I realize that isn't pertinent to this discussion exactly, just advice for people living in a country with way too many lawyers.