I'll go through the phrase piece by piece.
Latin has a few different ways to give a command; the most fancy/formal of these is the so-called imperative II. This is what's used in the famous phrase mementō morī: literally, "remember to die". I'd use the same here: mementō.
Latin has a few different verbs for this. Ālūcinārī is probably the fancier/more poetic one, but it's also a bit less positive, with implications of nonsense or foolishness. The more positive word, used for fantasies and prophecies, is somniāre. It's also more prosaic, coming straightforwardly from the word for "sleep", but I think it's what you want here.
…to know thyself…
Especially with the "thy", it seems like you're quoting the famous Oracle at Delphi. The classic form of the saying is gnôthe sautón, which is Greek rather than Latin—but later authors liked the saying too, and it's often quoted in Latin. The verb is generally either scitō or nōsce, while "thyself" is temet or tē ipsum; I like nōsce tē ipsum best, and that form was used by Hobbes and Linnaeus among others.
This form, of course, is a command, while you want an infinitive ("to…"). That form would be nōscere tē ipsum. (If you want to be really fancy you can use a poetic passive infinitive, nōscier tibi "to be known to thyself", which is more poetic and archaic—sort of like your use of "thyself" in English.)
The two main ways to say "and" in Latin are -qve and et. Here I'd use et.
…to keep above as below.
This sounds like a reference to Hermeticism; the famous Latin version of the Emerald Tablet contains this line:
Quod est inferius est sicut quod est superius. Et quod est superius est sicut quod est inferius, ad perpetranda miracula Rei Unius.
That which is below, is like unto that which is above. And that which is above, is like unto that which is below, in order to carry out the miracles of the One Essence.
(The translation is my own, so the "like unto" is just me being deliberately archaic. But when you're translating the words of Hermes the Thrice-Great, a bit of archaicism seems appropriate.)
"To keep" doesn't really have a straightforward Latin translation, so I'd use the general-purpose verb faciō, which is literally "make" but has dozens of meanings sort of like English "do". All together, this would be facere superius sīcut inferius. (The Tablet uses the singular; if you want it plural for whatever reason, that would be facere superiōra sīcut inferiōra.)
All together now:
Remember to dream, to know thyself, and to keep above as below.
Mementō somniāre, tibi nōscier, et facere superius sīcut inferius.
(Remember to have dreams, to be known to thyself, and to make that above like unto that below.)