Well, why are you trying to improve it? You going to FNMs? Or just having trouble beating a buddies deck?
I would say this as a piece of advice from a long time player to a fairly new one. A lot of newer players get into mill decks because they look fun on the surface and they are, it's an interesting win condition and even some control decks can employ it as a win con in a competitive environment (though not too much lately). However, these decks usually have to rely on control to stabilize the game because as you said they usually don't have a lot of life gain to help them stabilize against agro or any deck that does damage really. If you're not really attached to milling as a theme then I'd suggest going for a more reliable win condition.
as for improving the deck as is. These kinds of decks need to control the game to win. You can't aggro your way to victory with hexproof and cipher, people will eventually catch on if they haven't already and run cards that just beat you.
Lands - 28 is a hair to much, you should be able to go down to 25-26 tops. Get some shock lands (water Graves) and check lands, (drowned catacombs). Also a couple nephalia drownyards are good.
Creatures - You have some synergy here with the creatures you picked, I might get a hold of one aetherling as an alternate win condition, just in case you can't mill them out. Notion thief is okay, but in a deck where you want to mill them out, letting them draw is actually a good thing. All your doing is buying them more time. I get the combo with invisible stalker and cipher if it's working for you keep it. But i'd suggest auger of bolas instead of the fog bank. he can dig for cards for you and still block a little.
Spells - I really feel you can ditch most of the other spells in the deck. I'd like to see more control spells and less janky combo spells. get rid of the keyrune.
thought scour is a draw card you need to be running or at the least pilfered plans. Drawing is your lifeblood in a deck like this. and not from notion thief.
Tutoring is just bad, it's basically paying 4 extra mana and doing almost nothing for a turn to play a spell from your deck. Where as if you just had better drawing you could draw what you need.
take mindgrind down to 1 of, you only need it as a finisher, that's if you don't draw jace.
a jace architect of thought would help a lot against aggro and probably net you some more cards, Tammyio, moon sage is good in these decks too.
Get some more removal, and counters, run a couple syncopates and/or dissipates. It stops recursion effects like unburial or flashback from the yard. Get some doom blade in here. Dimir charm is really good, it has a lot of different modes.
Other thoughts - really to combat the life gain problem, you need white in the deck. White gives you access to so much stuff, supreme verdict, sphinx's revelation, blood baron, obzedat.
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