Picking references can be a tricky proposition. You want to find people who say good things about your work, but maybe it’s been awhile since you’ve been in touch. Or maybe you had a bad relationship with your manager at one of your important jobs; should you put that name down?

It’s important to get references right because of the weight employers tend to put on them. “Employers usually check your references as a last step before they make you an offer,” says Cheryl Palmer of Call to Career job coaching. “Because employers are looking for ways to screen out unsuitable candidates, this last step, which is the reference check, will either seal the deal or break it. You as a job seeker can’t afford to get this wrong.”

Here are her tips for getting it right.

Who to Ask

To identify good references, look over your career and pick a mix of former bosses and co-workers, Palmer says. Pick people who you worked closely with and interacted with often. “These are the people who will know your work best. They’re in the best position to convince employers that you are the best person for the job.”

There are some exceptions, of course. Don’t pick someone to be your reference if you had a bad relationship with them or if you are not sure what they will say about you, Palmer says. Avoid picking relatives, as employers assume they’ll be biased in your favor. And don’t tip off your current boss that you’re looking by asking them to be a reference.

How to Ask

It can feel awkward to ask someone to be a reference, but it’s important not to put it off, Palmer says. “I recommend that job seekers contact their references as soon as they know that they’ll be embarking on a job search,” she says. “That gives job seekers plenty of time to prep their references before their references actually have to speak on their behalf.”

Palmer says it’s best to pick up the phone when asking potential references to serve in this role. “A call is more personal than an email, and it’s a good way to catch up if you haven’t spoken to your references in awhile.” You can also offer to return the favor if appropriate, she says.

What They Should Say

Once your reference has agreed to speak on your behalf, they’ll need some information about what to say. If it’s been some time since you worked with your references, you may need to update them on what you’ve been doing as well as what your target job is, Palmer says. “A recent copy of your résumé can fill in the gaps for your references, and vacancy announcements that are representative of the type of position you are seeking can give your references more specific information about what aspects of your background they should highlight when talking with employers about you.”

Let your references know when an employer has asked for references so they can prepare for the call, Palmer says. Always let them know when you accept a job offer, and thank them for their help.