So much depends on the specifics of the client's situation. That goes without saying, but shouldn't go without saying.
Most importantly, regardless of the situation, be extremely careful with regards to incentives. This is thin ice with almost any review site that matters. My advice would be not to do it at all.
For most businesses getting 1 review a month would be plenty and, most likely, a huge improvement. With that in mind I would try to create a system/process for the long haul, rather than treating it as a campaign. The only exception to this would be a brand new business or a an established business that had almost no online reviews.
The most challenging thing is getting everyone on the team to own it. This has to be "baked in" to the culture, the systems, the physical environment, etc... (Which remind me, you could incentivize the team to get reviews.) In some situations (re: larger scale) it might be worth using a 3rd party solution that handles the customer follow-up (e.g. GuildQuality.com). For most, I suggest keeping it personal. Have requests come from the owner, store manager or sales person.
In the end, if the service or product is actually bad, then best of luck to you. There's no way to overcome that. Sooner or later the truth gets out.
Feel free to contact me to discuss a specific situation.