Teaching through everyday activity, observations and play is likely most effective at this time.
Generally, we (preschool teachers) usually can get about "one minute per year" focus on anything in specific via direct instruction, concentrated focus is hard to maintain, so avoid dumping lots of information. Find a way for the child to interact with what you are trying to teach.
Keeping it playful and short is key at this age. Also remember not to 'quiz' or 'drill' kids... that sense of parental expectation and repetition can quash their joy in learning. Flash cards, too... too much expectation. Playing with children, counting toys, playing with letter shapes (they can be so many things-- cookies at a bakery, letters for 'alphabet soup', "can you make a picture or shape with them?") talking about colors as you put them onto paper or use food coloring in water... all of these are great ways for getting kids engaged. The more they are engaged, the longer you can stay "on topic".... just remember, the kids are sometimes discovering other things *they* notice than what is important to us as the teaching adults, so be sure to give their discoveries attention, explanation and *your* focus too. Kids learn better if they 'discover' the knowledge on their own sometimes.