Question: I read the Wikipedia article on [DNA methylation](https://en.wikipedia.org/wiki/DNA_methylation)

Let's say I want to extract and then stock my current DNA methylation marks somewhere so that I can use it safely 20 years in the future for a medical procedure that doesn't exist yet.

What method should I use?

Answer: 
To record the current methylation state of your DNA, you can use [bisulfite sequencing](https://en.wikipedia.org/wiki/Bisulfite_sequencing). Basically, you take half of your DNA sample and treat with bisulfite, which deaminates cytosines (C->U) , so they read as T instead of C. Methylated cytosines are protected, so they still read as C. You run two sequencing reactions, one with bisulfite-treated and the other with untreated DNA, so you can tell where the true Cs are in the genome. See the below diagram:

[![bisulphite sequencing image](https://i.stack.imgur.com/8UPQt.png)](https://i.stack.imgur.com/8UPQt.png)
[Wikimedia commons](https://upload.wikimedia.org/wikipedia/en/c/c9/Wiki_Bisulfite_sequencing_Figure_1_small.png)

Note that if you're thinking of doing this from a longevity point of view, that there are [hugely different methylation patterns in different cell types](http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3759728/); I'm not sure which cell type you'd want to pick. Also, methylation is not the only covalent modification of DNA that exists, there's also [hydroxymethylation](http://www.ks.uiuc.edu/Research/methylation/), and I'd bet there are more that we haven't discovered yet. There certainly are in bacteria and phage ([glucosyl-hydroxymethylcytosine](http://www.ncbi.nlm.nih.gov/pubmed/26081634), for one). The hard thing about saving things for the future is that we really don't understand the full picture of what's happening now epigenetically, and as such, we don't have assays to detect things we don't know about.
I believe the DNA is going to be as stable in native form as it should be after bisulfite treatment. You can use commercially available methods of storing DNA samples and safely assume it will be intact in 20 years time.