Formal verse appears vulnerable to seeming archaic because it has generally fallen out of fashion. It may also increase the sense of immaturity, particularly with strict following of obvious forms (perhaps from a singsong effect, perhaps from not expertly breaking the rules).
While I rarely object to and have sometimes fully embraced an archaic sense, there may be times when a more modern feel is desired. Since a juvenile feeling is commonly associated with lack of quality, I would like to avoid it more generally.
Obviously reading good modern formal verse would help, but such verse is less common and less well filtered by time. Years ago, when I was more interesting in poetry, I read some Robert Frost (which sometimes seemed a little archaic because of its rural emphasis) and a little Edna St. Vincent Millay (which I enjoyed less, perhaps because of a more modern sentiment), and works in The Lyric and Plains Poetry Journal (to which I subscribed in part because they were likely targets for my own writing).
While I would certainly benefit from more extensive reading, I am interested in what techniques can be applied to avoid a juvenile or archaic feel without abandoning the creativity-enhancing constraints and aesthetic quality of formalism.
Techniques which seem to have been somewhat helpful include the use of longer lines, non-iambic feet, and more complex rhyming patterns as well as the mild violation of strict formal rules. Vocabulary and setting might also play significant roles in the feel of verse.
Obviously, what the verse is seeking to communicate introduces constraints, but within such constraints there remains significant flexibility.
Sentimental, idealistic, and playful inclinations may exacerbate this issue, but I do not wish to exclude such from my writing. (I am a something of a sentimentalist by Oscar Wilde's definition: "a man who sees an absurd value in everything, and doesn’t know the market price of any single thing"—Lady Windermere's Fan, Third Act.)
Incidentally, I have enjoyed playing with less familiar forms such as analyzed rhyme and alliterative verse (as from Sir Gawain and the Green Knight), so I am reasonably open to unusual forms.