Step 1: Support from the most senior person onsite. This is either super easy or incredibly difficult, there is no middle ground. When it's super easy it's often a matter of opening their eyes for them, they are consumed with very different issues than we as quality professionals are on a daily basis. Set up a meeting, bring your external audit 'hits' from previous audits, bring your existing internal audit 'hits', show him/her the similarities between the two. Chances are you have some internal audit findings in January that went uncorrected that you also got hit on for your customer audit later that year. (C'mon, we can't be only place with that frustration?) This should anger your most senior person onsite, and when the most senior person onsite gets angered those under him/her suffer. When those under him/her suffer they are forced to do what you ask them to do. Yes, this is the equivalent of being a schoolyard tattle tail but sometimes you have to go there.
Step 2: Develop an internal auditing checklist. It's been mentioned in previous comments. You can copy the items from whichever standard you are using or you can look at what your current issues are develop a checklist to look for those things. For example, if you know following GMPs are a challenge for your employees you need to have some specifics on your checklist to look at your GMPs. If you know propping doors is a problem, put it on the checklist.
Step 3: Develop internal audit training. We use a powerpoint presentation that includes some audit tricks like count the F's in this sentence: Finicky felines are the result of years of scientific research, combined with years of experience." Most people won't see the F's in the word 'of' because our brains skip over those little words. Unless of course English is not your first language in which case you will probably notice them because you are paying closer attention. The training includes the purpose of the internal audit activity, use of your senses, and some tips on what sort of things to look for. The powerpoint presentation is followed by an on the job training with someone who has done the auditing to show them how to do the audit and use the checklist.
Step 4: Create a schedule. I have an overcomplicated spreadsheet with the areas of plant, divided into 3 sections and spread those sections over 3 shifts each month. All managers, supervisors, lead persons and QA and sanitation personnel are subjected to internal auditing. They all hate me equally, except for those who have to audit the building exterior in winter, they hate me more. I work out the schedule for the first 6 months of the year and send it to everyone involved before the first of the year so they can plan my demise well ahead of time. I then remind them again at the start of their scheduled month and provide the audit checklist and the areas they are to audit. I remind them AGAIN halfway through the month so there are no excuses. Our plant manager has made it crystal clear that he expects these audits to be completed each month and insists that I tattle if they are not done. There are some people that I have to nag further but I know who they are and can mentally prepare myself for the 6 emails that I know I will have to send them.
Step 5: Communicate. I recently started posting internal audit results in each area. We have an odd group of employees who want to do a good job and take it personally when they see that they lost points for a GMP issue. I rarely see the same issue twice because they now are reminded each month that these things are important and that we are checking. If heaven forbid someone doesn't do their audit, there is an uproar in the plant because they didn't get their results that month and well, the issue of the person not having done their audit sort of takes care of itself when the line staff start questioning them. Communicate the other findings in a meeting with management either weekly or monthly, whatever works for you. Once people are held accountable for corrections, they will start to fix things. This is also why it's important to have a multifunctional audit team, if it's all maintenance personnel they'll stop reporting maintenance issues so they don't have to fix anything and if it's all production supervisors you'll never hear about GMP issues so they don't have to follow up with employees.
Anyhow... this got super lengthy... good luck!
“FINICKY FELINES ARE THE RESULT OF YEARS OF SCIENTIFIC RESEARCH, COMBINED WITH YEARS OF EXPERIENCE.”