What clothes to wear from Costa Rica -> New York
 

Hi,

I'll be traveling form march till June. What temperature is it going to be and what type of clothes do I need to wear while riding? Anybody any tips?

Where are you staring from ( oops your title suggests a Costa Rica start)and at what day in March?
Winter should be winding down in Mexico , spring will be busting out all over, still some chill nights in the high country of NW Mexico but otherwise it will be like Summer in Canada and warmer as the seasons progress.
If you can escape the winter of the north just dress like you would in summer in Ontario, California , Florida,
Dress lightly but safely and sensibly with lots of ventilation , carry some layers for the cool spells and have a rain suit to pull over for wet days.
If you hit late March I the USA you would be smart to keep an eye on the weather forecast and hug the coast along the
Gulf of Mexico and the Atlantic to stay in spring weather.
Really no need to spring for a super expensive heavy armor plated jousting astronaut suit with scores of pockets , buckles and zippers made of black heat- sink material or the multicolourded billboard for certified globe trotting .
I like riding in jeans, a mesh jacket with some minor safety padding and work boots. That way I can hop off the bike and blend into a crowd fairly inconspicuously and walk around in comfort .Add some layers as you go north in the USA.
I know , some say to dress for the slide , not for the ride, but if you ride sensibly your slides will be few, and not long at all .( been there done that) and
the light padding will be enough.

Early Spring...
 

March thru June and ending in NYC. I'm good at math, counting in this case...four months. Let me double check. Yup four months. The big questions in my mind, how are you planning to pace your trip? Take your time getting to the US and you won't have to worry too much about warmth. Although riding in 50F weather for hours will take its toll on you if you don't have sufficient gloves, socks, the works. April and May can easily have cooler days, depending on your route. Are you planning to ride through the mountains or as Sjoerd mentioned hug the coast?

June 2013, the last leg riding home in 9 months on the road, I hit rain 5 out of 6 riding days. Soaking rain for hours. Just saying.

You probably won't encounter snow. Count on everything else.

Take layers and rain gear. That way you can dress for warm or cold, rain or dry, etc.

I would not ride anywhere long distance in jeans, thats just plain bad advice.You should have full safety gear on, a boot, pant with protection, protection in knees, jacket with elbo, shoulder and back protector, and full face helmet. Its all two lane roads overall, no shoulders, and rain in higher elevations in rainy season, which starts in April in Costa Rica.

If you see the way people drive in Central America, the more safety gear the better. Getting to a decent hospital could take hours and hours...

There is plenty of riding gear made for all the elements and temps. Alpine Stars. Klime etc,even Tourmaster and Icon, is better than nothing.