Economists recognize that monetary incentives can backfire through the crowding-out of moral and social motivations leading to an overall decrease of the desired behaviour. Under the premise that agents are heterogeneous and have various intrinsic motivations we suggest precise strategies to reduce counterproductive motivational crowding-out. In order to test our suggestions, we implement a field experiment where participants are asked to fill a questionnaire on pro-environmental behaviours under different incentive schemes, either with no monetary incentive (control) or with low or high monetary incentive directed either to the respondents (design A) or to an environmental cause (design B), or with a choice offered between A and B (design C). We investigate (i) whether there is a significant crowding-out effect, (ii) which design performs better to promote participation. Except for a high monetary incentive where the respondent chooses himself the end-recipient, we show that monetary rewards directed either at the individual or at the cause actually harms intrinsic motivations, but not to the same extent.