This isn't that much different than what I've listed here in the past. Here are what I consider are the fundamentals for effective supplementing.
It all starts with a thorough inspection with lots of pictures showing all the items that potentially should be paid for. It should show pipe jack boots, vents, valley metal, drip edge nailed down on top of shingles and felt. It should show any damage or deterioration of those items. It should show if mastic or caulk have been applied to those items. For most items, it is not possible to successfully supplement without pictures to illustrate and factually prove your request.
We attempt to go to every adjuster meeting with an estimate in hand. The most effective way to supplement is to do it before the scope of loss is written thereby eliminating the need to supplement. I realize the vast majority don't do this and I've heard all the arguments for not doing this (which I disagree with by and large), but we've found this to be reasonably effective. And why does it matter? If we're going to do much supplementing, we're going to have to put together our estimate at some point anyway. Why not do it upfront where we can hopefully use it to get more items approved with the initial scope of loss. I'd rather be proactive than reactive. Our guys having the Adjuster Meetings are supposed to ask every Field Adjuster who we will supplement with and where we will send the supplement. The response we most often get is "What do you think you'll need to supplement?" to which we respond "All the items on our estimate that you don't approve." That sends a very clear message. Most Adjusters would prefer to close the claim asap. We're making it clear there is one way to accomplish that. Down the road, when we are supplementing, we can say we presented these items upfront and they were ignored. This works quite well with many of the regular, local adjusters we deal with. They learn in a short time that we aren't kidding and their actions have shown they'd prefer to reach agreement, on the spot, with our Representative versus a protracted battle that often means getting the Homeowner involved and the HO's Agent.
In the event all this fails, we want to supplement promptly after receiving the scope of loss. When possible, this is done with an email with documents and pictures as attachments to prove our points. I would suggest you create some email templates to use as most of the items you are supplementing for come up time after time after time. Send the email or fax supplement in, give it 24 to 48 hours, then call. Keep following up, in reasonable intervals, until you get to speak with either the field adjuster who handled the claim or a desk adjuster assigned to the claim.
If you do not get responses and/or if reasonable factual requests are denied, it's time to get the Homeowner involved. If your Sales Reps are doing their job properly, they've set the HO's expectations properly so they know they may need to call their Agent, call the Adjuster, call the Desk Adjuster or call the Desk Adjuster's manager to get things done. An irate Policy Holder calling a SF Agent often works wonders. You, as the Contractor, have zero legal standing with the insurance company. If the insurance company wishes to attempt to reconcile the claim, so you can proceed with the work, great. If not, you need to get the HO involved as they do have a legal standing as the contract/policy holder with the insurance company. I cannot emphasize enough the value of involving the HO in supplementing efforts.
Now let me digress momentarily. Your contract should have clauses in it that state items not approved by the insurance (such as vents, pipe jack boots, etc. - we sometime call this our "State Farm Clause") and/or items that are required by local Residential Building Code that are not covered by the insurance the financial responsibility of the Homeowner. Guess what, if the HO suddenly finds they may be coming out of pocket for an additional $200 to $1000 or more, their motivation to get involved goes up exponentially. Why do you think SF quit paying for PJ Boots, vents, etc.? IMHO, it is because they realize in most cases, the roofing contractor will go ahead and cover these. That's great, right? HO gets a complete new roof, SF pays tens and hundreds of millions less for roof claims over the course of the year and the Contractor eats up their profit. This is another solid reason for why you want to get most supplementing approved prior to the work being done. If the HO has an issue with it, then just tell them you will reuse the old vents, pipe boots, etc. and will not warranty any of these items. If you've set expectations properly, you won't run into many problems. If you don't set expectations properly, shame on your Sales Reps.
If you employ these strategies properly, and are patient and persistent, I think you'll end up getting 75% of your supplements approved, if not more. If you're looking for 100%, pinch yourself to wake up. There is no such thing as 100%. However, if you take the average scope of loss, you should be able to increase it 20% to 50% or more with proper supplementing techniques and processes.
Let me add at this point that IMHO, you're also best served to use Xactimate and become as skilled as possible in utilizing this estimating software. While I know there are plenty of detractors, if utilized properly along with proper supplementing techniques, roofing contractors can do just fine from a profit point of view.
If all this fails, and if done properly it rarely will, you always have the option of suggesting third party appraisal or a Public Adjuster (in States where it is allowed) as a means to getting the claim properly reconciled. There are great PA's, good PA's and those that suck. Two guys that used to post here often, Ray C in TX and Dave S in IL, are off the charts good. It most often takes a fair amount of time but you won't believe what these guys can get done.
I cannot possibly cover every little trick of the trade in this forum, I don't have the time to write a book. Follow these principles and you should see big improvements. Purchase the appropriate International Residential Code Books (I do it in ebook format so they're easy to reference as well as copy and paste). Understand where the codes apply. Remember that you're dealing with multi billion dollar conglomerates that don't do well with verbal BS and stuff scribbled on a napkin. Provide the person you're working with the factual reasons, evidence and documentation to approve your line items in order that they can justify it to their management. Good luck!