What is most useful for improving the performance and behavior of less skilled staff?

I think..
You need to understand the root of a performance problem before you can fully address it.
Ability and motivation go together to impact performance, and the most successful performance improvement efforts combine strategies for improving each.
This creates a positive environment where people feel supported to reach their performance potential; and feel valued, knowing that the organization wants to find a good fit for their abilities.
At times, your interventions may not be enough to salvage the situation.
As long as you've given performance enhancement your best effort, and you've reasonably exhausted all your options, then you can feel confident that you're making the right decision if you do need to let someone go.
Before going down that route, however, ##try the strategies discussed here and create a great work environment for your employees – one where their abilities are used to their full potential, and where good motivational techniques are used on a regular basis##

Focus on discussing their strengths rather than their weaknesses.
Cite a situation from the past wherein he/she was involved or contributed on an activity or a project that became successful.
Instead of trying to improve weakneses, focusing on employee's strengths brings out the best from what is already there - and is usually has immediate effects

The most useful for improving and behavior of less staff skill is SUPPORT.

Appreciating, Thanking and acknowledging them on every steps is the best keys perhaps words!!!!! The best encouraging words are " Remember, You are twice as good as you think, You are!!!!!! Make them feel important, and always stand by them till they earn the skills and you learn to teach.

T R A I N T each - lead by example.
Staff always look at the qualities of others they admire.
Mentorship useful.
R espect - goes a long way and achieve more from workers who are motivated and feel valued.
A ttitude - demonstrations of good work ethic, work relations and standard of work = higher work productivity.
I nterest - encourage development of skills, taking advantage of training/educational opportunities and investment in future business plans and where the workforce fits into those plans.
If workers feel part of something important can spur them on to step up their game and be competitive in a good way.
They will be encouraged to think seriously about what they can bring to the table in terms of what they have to offer the company and could offer in the future.
N egotiation - communication the mother of all work performance and behaviour issues.
Oppressive environments to be resisted and not tolerated, only breeds discontent and poor level of performance and production.
A work environment where good communication is the standard will foster higher standards of expected performance and behaviour.
Workers like to feel heard/listened to.
'Open door' for issues to be aired is paramount to discussing and resolving issues and concerns swiftly and to the satisfaction of everyone concerned.
Most skills and behaviours can be improved upon, the only way is up in this regard.