This is an item from Anton Moker's Decalogus metricus et paraenetica disticha, published in 1573. You can see the book at GoogleBooks.
Diu Meditare
Non in consilio sis praeceps; tardior esto:
Sunt meditanda diu quae statuenda semel.
The vocabulary is keyed to the DCC Latin Vocabulary list. There are two words in this poem that are not on the DCC list:
meditor, meditārī: consider, ponder
praeceps (praecipitis): headlong, steep, swift
cōnsilium -ī n.: plan; council, group of advisors
diū: for a long time
in: in, on (+ abl.); into, onto (+ acc)
nōn: not
qui quae quod: who, which, what / quis quid: who? what? which?
semel: once
statuō -ere -uī -ūtum: set up, determine
sum, esse, fuī: be, exist
tardus -a -um: slow, sluggish, lingering