Wraith Bands: Slippers and Circlets are taken early-game and turned into Wraith Bands later for cheap stat-increasing items. Wraiths help to increase your damage and HP slightly, all great for Mirana's needs. Wraith Bands can be stacked if you feel like you need the cheap HP.
Magic Stick: Can be upgraded to a Magic Wand if you feel like you need the additional, no CD charges.
Boots: Depending on the circumstances, a form of upgrade to Boots of Speed should be taken sometime during the core build.
Boots of Travel: BoT are so great on Mirana that some go as far as to call it core. It provides priceless mobility and really complements her role as a ganker with great map-presence. However, unless you're farming extremely well and can get a quick pair of BoT, leave them until later on in the game as they delay your core too much and leave your stats gimped.
Phase Boots: A viable upgrade to BoS on Mirana, Phase Boots make chasing a fleeing target with Elune's almost a joke. The damage and armor it provides is nothing to be laughed at either. Phase Boots are a viable option on a heavily gank-oriented Mirana.
Treads: Treads are more suited for a team who is relying on Mirana to carry the game should it extend that late. The ASPD bonus is great, and the +10 STR really gives Mirana's low HP pool a boost for greater survivability. Almost always, you'll want to keep Treads on STR. Treads are recommended over Phases for most games, as the IAS and STR is great for Mirana, but there are a few circumstances where Phase Boots may be more beneficial.
Bottle: Never start with a bottle; instead, have the team courier ferry it over to you as soon as possible. Bottle is great for Mirana: it gives her burst regen and the ability to capture runes, all suited towards her role as a solid ganker.
Diffusal Blade: Diffusal Blade turns Mirana into a ganking goddess (sorry, Priestess), as well as gives her cheap stat bonuses, feedback, and most importantly, Purge. Purge can be used to remove buffs from enemies like God's Strength and Insatiable Hunger, or to remove negative buffs from allies like Track. Most of the times, however, you'll be using it for the slow that makes enemies crawl at a snail's pace.
Tip: Diffusal Blade should always be taken should the enemy have a significant purgeable skill (Sven, Broodmother, Warlock, Omniknight.)
Scroll of TP: Self-explanatory, you should always have one-two of these on hand for quick teleports. TP to counter-gank, TP to defend towers, TP to gank, TP to farm, and TP to safety. Irregardless of whether you think you need it or not, you never know when one will either save you or net you a kill, and as such, should always have one on hand.