Which things are important to motivate construction site labourers?

Motivation for site labors depend on productivity rates at the site by making a certificate of award to the best crew in the productivity report , also we have to increase their skills by training and safety knowledge 

I would tell them to comply all the safety requirements like the PPE's. Safety first before anything else for their own good.

Employee Welfare is what is of paramount importanc.

1. Drinking water
2. Good Food (if provided)
3. A Communication channel to raise grievances
4. well planned trnasportation which ensures they do not spend much time unnecessarily in buses (most buses are non-AC)
5. Timely medical attendtion when required for incidents
6. A Safety Officer to ensure they wear PPEs etc and follow safety rules

Now i am mindful that a lot of people might think these are not really very necessary.  i personally feel if this is done, they will be a very happy workforce ad will do whatver is required.

i chose to write the above having witnssed conditions otherwise in a lot of companies.  Seen them literally begging for first aid and visit to a hospital, drinking water during summers etc.

Not to be very strict to them, give them some space and appriciate their good work. Arrange trainings for them, explain the work very well. Give over time and increments.

By being freindly to them  just like your brothers at home so that they will respect you when there are correction of their work base on SAES.

Yes, it is very important and must be do it by lead in charge to motivate the labor force, because the production of each project base from the accomplisment which is coming from the labor, the lead or the engineer is to give or guide what todo, in easiest and safety manner.