So true, there is nothing as frustrating as getting that call from a candidate who recently signed your offer letter and employment contract. 

It's the kind of call that all recruiters and hiring managers hate to get. It usually goes like this: “Sorry Khun Tom, but I don’t think I can join your client. I know I already signed the employment contract and that I promised I would never change my mind. But you see, my boss has given me a new big important project. He told me I’m the only one in the company who she can trust to lift this sort of responsibility. They are all so nice to me. And she also gave me a new title.”  

Me (after she hung up): Aaaaarrrrggghhhh !*$#@#$*!!!

There is on-boarding and there is pre-boarding. The latter, pre-boarding, are the things you do during the interview process. You should ask questions like why the candidate came to see you, what attracted the candidate to the position. Check how the candidate will handle a counter-offer from their current employer and it will give you an important hint as you try to assess the risk of losing a successful candidate in the last minute. 

The on-boarding starts the day when you and the candidate sign the employment contract. Don't even think that you are home and dry. Believe me, you are only half way. You need to fill the time from the signing of the contract to the first day of employment; in fact even weeks into the new job. 

Here's a list of activities: offer a show-up bonus, have frequent telephone calls with the person, invite for lunch or dinner with the new colleagues, ask the candidate to call you immediately after giving notice, set up their new email and let them access, print the new business cards and send them to the home address, include the person in your WhatsApp or Line group. Be creative and you will have deserved the champagne when your candidate turns up day one.