Let me share my experiences from my own M&E assessments.
When designing questionniares it is important to already think through the culture and farmer requirements. For example if its in rainy season, the farmers will usually be very busy with farm work. It is best to have an initial visit to plan with them in advance. This is where you go through a farmer representative to do the planning.
If survey targets women allow them to bring their toddler also make sure its not schedule around the lunch hours
When conduct focus group discussion its better to meet them at their usual meeting hours than requesting for an extra ordinary meeting.
The questionniare should be as concise as possible.
Administer the questionniare in their local language. This saves on translation time.
Explain to the farmer the purpose and expected result as well as expected time of the interview and seek consent for the interview.
If they are focus group discussion ensure that they use alot of participatory methods other than just questions
If its field based activities, ensure that the farmer takes part in other activities such field measuring. It motivates them.
Train the enumerators. Let them understand the meaning if each to avoid redundancy
On incentives, we always try to provide the respondents a drink