How to Trek in Annapurna During the Monsoon

You can totally trek in the Annapurna Conservation Area without a guide. In fact, I said as much in the story of the third day. Indeed, I met up with other travellers – including one who trekked solo to Poon Hill – who went in without a guide. 
And this is fine if nothing goes wrong. However, the value of a local guide comes in when things do not go fine. Numerous times in my trek, it was made more comfortable or safer because of my guide's local knowledge and preparation.
The monsoon brings additional challenges to the trek. She could get near real-time news from guesthouses, locals and other guides on the condition of trails, and whether the monsoon had made any of them impassable. She could range ahead for help if needed, and could call for help more effectively.
The higher you go and the further away from the farmed areas of Annapurna, the more it is a good idea to trek with a guide. Especially if you are trekking solo. 
Besides, it's much more fun to trek with a local! And it is a way to make your trip more sustainable, by supporting local guiding professions. In my case with the 3 Sisters – female livelihood as well! 

Put as much time as you can afford to the trek. Aside from the monsoon rains making the trails more of a drudgery, you'll want to linger in the amazing fantasy forests and alpine summer plateau – and you should. You'll want to linger in the guesthouses and perhaps browse a little bit, take photos, try yak cheese – and you should. 
Give yourself time to enjoy the things along the way to the mountain. 

Whether this is a portable water filter, which is what I did, or a Steripen or chlorine tablets, there's nothing so empowering as this.
The water filter is so small and light, it's nothing in the backpack, and it is so fast that it is literally more convenient than finding a shop to buy bottled water. It is!
Even if I weren't already actively avoiding plastic bottled water, it literally made the chore completely redundant. 

I mean, I had wanted to take the elderly guide's advice and offer, in Pokhara. But it was kind of late to be changing boots, and mine did the job after all. But, indeed the soles were slippy on the wet rock crossings because they were not really meant for trekking – light hiking, maybe.
Good boots is generally good trekking advice anyway – it's just that with the monsoon river crossings when you could encounter raging streams and fast-flowing waterfalls, the advice goes double. 

The monsoon summer season is almost warm enough for me to trek all the way up to the base camp, wearing thermals and layering as if for winter, for the upper levels. If you stay at the better guesthouses, in the monsoon you should not be competing for beds – and most importantly, blankets. Double up on blankets at high altitude and you would not need more than that. Maybe bring a thermal sleeping liner – much less bulky and really quite warm. 
Except if you're as intolerant of cold as me, in which case you will need that thermal sleeping bag on the highest altitude night at the base camp.
But my point is, even I only needed it for that one night.  

You could stay reasonably dry from top to bottom with good waterproofs and if you brought truly waterproof pants (unlike me). At worst, you could avail yourself of the local bin bag poncho and skirt solution.
But you will certainly have wet socks, no matter how good your boots are. Nothing makes you more miserable than not having dry socks to sleep in, even if you could bring yourself to put the damp socks back on for the trek every day. 
However, dry socks – even if only temporarily dry – is such a mood booster in the monsoon, for such small things!

One of the things I was paranoid about, was running out of battery on my camera phone, and being unable to record incredible things (because as we all know, according to Sod's Law, the incredible things will happen subsequent to your camera being out of battery). I once encountered a majestic school of bumphead parrotfish, stately as a herd of mastodons, right after my snorkelling camera's battery died. 
I was not sure about whether I could easily recharge either phones or power banks at the guesthouses. Long story short, I brought three power banks, not including my flashlight which could also double as a power bank. And a universal adaptor that could take several USB connections, in case I needed to share the socket with other travellers. 
That was… overkill. In the peak season, it may not be. But in the monsoon there is a lot of vacancy in the guesthouses. In fact, some guesthouses don't open for guests at all in the rainy season. The guesthouses I stayed at also have universal sockets already, so you don't actually need to bring an adaptor.
A couple of power banks is enough, for contingencies and convenience. Just one, if you're cool with not really taking pictures.

However, do make your primary camera as waterproof as you can. The monsoon can be relentless with rain, and you won't be able to record any of the gorgeous sights if your camera cannot cope with the wet.

Really, whatever medication or salves that you need to keep your joints and muscles going, bring enough of it for the whole trek, and then some. Otherwise, be prepared to re-supply at Chomrong, as the other pharmacies may not open in the monsoon. 
For myself, I really ought not have underestimated the effect of my trekking diet on my knees. It made the last three days of the trek much less enjoyable than the way up. 
Also, vote yes on good alpine trekking poles.

Now, I'm not saying you need to trek in a Hazmat suit. However, similar principles apply. Tuck tops into pants, and pants into socks. That sort of thing. The leeches will get in if you don't. They are resourceful little buggers. Be prepared to flick them off your clothes, hair, forehead, and from inside your bra.
They're pretty much harmless though, just super annoying and a little bit itchy afterward. Note: if they have already fed and fall off on their own, it's supposed to itch less. My skin is not very sensitive so I've not really noticed a difference.