With the rise and fall in value of bitcoins, how do I store them once I've accummulated enough With the rise and fall in value of bitcoins, how do I accumulate the value so that I don't continue to lose them?
There are a couple of different aspects to your question.

How to avoid accidentally losing coins?

You need to have reliable and secure backups of your wallet's private keys.  Depending on the wallet you use, this could be a file (e.g. wallet.dat), a mnemonic phrase, or some other sort of information.  Make sure you know what you are storing and how to use it to recover your wallet.  In particular, just storing addresses is not enough.  If possible, practice your backup and restore procedure, even if you have to make some unnecessary transactions (and pay fees).  

Keep the backups somewhere safe, perhaps in multiple copies, so that no single foreseeable disaster can destroy all of them.  Also, the backups must not be accessible by anyone untrusted, or that person could steal your coins (see below).  You could encrypt the backups, but then you need to figure out how you will make sure to remember the encryption key or passphrase.

You can store your coins with some exchange or online provider (Coinbase, etc), but this carries risks: they might go bankrupt or lose your coins due to mistakes, fraud, theft, or accident.

How to avoid having coins stolen?

We have another question about this: How can I keep my wallet secure?

You have to make sure that nobody else ever gets access to your private keys.  If you store them on the computer, phone, or other device that you typically use to work with Bitcoin, be sure to account for the possibility that the device may get lost or stolen or hacked.  Use whatever encryption or other protection mechanisms are provided by your wallet software or operating system.  Use good security practices in general; avoid installing unknown software or visiting unknown websites, and keep up with security updates.

You can also store some of your coins offline, in a paper wallet or offline USB stick.  You'll need to make sure this is stored in a safe and secure place, where nobody else can get to it, and be sure to have backups in case it is accidentally destroyed.

How to avoid having coins lose value?

You can't.  The price of Bitcoin will continue to fluctuate.  The only way to "lock in" your gains is to sell some or all your coins (or perform some sort of exotic hedge which amounts to much the same thing).   This does mean that you won't benefit (as much) if the value goes up later; you can't have it both ways.