It’s always a smart idea to offer to write a letter of recommendation for a colleague who you know is well qualified for the job in question. Consider it good career karma; after all, you never know when you’ll need a letter of recommendation for yourself, and your colleague may be happy to return the favor.
Be specific about the candidate’s qualifications and successes. Describe what the person has done using specific instances and examples, rather than generalities. Whenever possible, give evidence or stories to back yourself up.