Now, I'm well aware that most people don't play VNs twice. Visual novels are a static media, similar to one of the old 'choose your own adventure' novels in interactive terms, so this is only natural. To be blunt, the main reason I go back and play old VNs is because nothing is satisfying one of my itches amongst the more recent releases. That said, there are some pieces of advice I can give for those who habitually re-read their favorite books and rewatch their favorite anime.
1- Wait long enough for your memories to fade: The human brain has a tendency to 'compress' old memories, and it is rare person who, through training or at birth, possesses an eidetic memory. As a result, details do fade over a period of time that tends to vary greatly with the individual. In my case, the base runs from a year to a year and a half for VNs that made a good impression and four months for ones that didn't.
2- Pick your paths: When it comes down to it, most of us are going back for a particular heroine or path. We aren't that interested in rehashing the heroine paths that we didn't find that interesting, and this is only natural. Sagaoz and other sites with complete saves can let you go to the true ending without bothering with the heroine endings, if that is what you want.
3- With gameplay hybrids, make full use of your save data: Most VN hybrids have NG+ built in, and as a result, you can breeze through the game portions of most of them rather easily by simply using your own save data. This is immensely helpful in games with a particularly tedious bent (like srpgs), where re-leveling would take forever.
4- Limit replays to your favorites: While I occasionally get a junk-food-like craving for something crappy that nonetheless remained in memory, in most cases I only really enjoy replaying my favorite VNs (in my case, a list of about fifty).
5- Nakige and utsuge work, but pure charage don't: I'm not kidding. Pure charage are agonizing to replay, no matter how long after you go back. I can still cry for the sad scenes in a Key game, but if you asked me to replay anything by Feng or most games by Navel, I'd rather cut off my balls and hang them out to dry on my windowsill.
6- If you fall asleep, just stop- In my experience, nothing is worse than getting bored of your favorites and then forcing yourself to continue. If you can't pay attention or if you suddenly lose interest, it is time to stop. If you force yourself to continue, there is a distinct possibility you will ruin your own impressions of the game in question for future playthroughs.
7- Stay away from pure mindfucks- I shouldn't have to explain this, but I will... the value of a mindfuck is in its surprise. Games centered on a mindfuck, with the sole purpose of trying to fool you into thinking one thing while something else is going on, are terrible for VN replays. This is because they are probably the most spoiler-vulnerable genre out there.
8- Highly emotional or intellectually stimulating works will often gain more depth: This isn't a fanciful statement. In my experience, a VN that is trying to get across something else besides pure story or something that is trying to make you cry will inevitably make for a better replay than something that is just shoving sex, romance, and comedy in your face. I could probably replay Houkago no Futekikakusha, for instance, three or four times in a year without the emotional aspects fading significantly, and I find new things out about Dies Irae, Vermilion, and Devils Devel Concept with each playthrough.
9- Infodumpers take longer to recover from: Bradyon Veda, I/O, Muramasa, etc... VNs that infodump seriously as part of the storytelling tend to leave a lot of info inside your brain. As a result, it takes significantly longer for your memories of them to fully 'compress'. Don't expect to be able to enjoy anything with frequent infodumps at less than one and a half times that of any of your other favorites.
10- A good night's sleep is your friend: Why am I emphasizing this? Because to get the best out of a truly great VN, a well-rested body and brain is necessary. Nothing kills enjoyment of a good story like being unable to grasp it due to brain-numbness from sleep deprivation.
Hope yall enjoyed my little lecture, lol.