1. Teach Skills Not TraitsRather than trying to change the personality of the individual, focus on training skills that can be taught and learned.
For example, suppose you're responsible for a field engineer whose duties entail going on customer calls. If she is naturally introverted (a trait) don't try to convince her to be more extroverted (a trait) in order to help you sell. Instead, train her how to listen actively (a skill) and how to use terminology customers will understand (a skill).
2. Teach the Appropriate SkillOnly teach employees skills that you're certain will produce tangible results, within the context of that employee's job.
For example, if a sales team consists of hunters (who find new business) and farmers (who develop existing accounts), it's wasteful to train everybody on the team on cold-calling techniques. Limit such training to the hunters and provide training in other skills (like account management) to the farmers.
3. Reinforce and Support the SkillWhenever you train a skill, provide multiple opportunities to check on how well that employee is executing that skill and provide coaching as necessary.
Learning a new skill entails making it into a habit. Unfortunately, doing so usually involves overcoming existing habits, which is inherently difficult. Coaching allows you gradually reinforce the skill and overcome the habits it replaces.
4. Implement Skill-based MetricsThere are no truer words in business than "What gets measured gets done." If you really want employees to integrate a skill into their day-to-day performance, you must, must, must measure the results of the application of that skill.
For example, if you're providing training on some aspect of your sales process, you should measure the conversion rate at that stage of the sales process, rather than just measuring the total revenue that's booked at the end of the quarter.