It is obvious that pesticides play a very important (negative) role. For example also professor Randolf Menzel, one of the biggest bee experts worldwide, is convinced of the negative effects of neonicotinoids on bees. So we all should, wherever possible, avoid the use of pesticides and demand to restrict them from our governments.
Then single-crop farming and sterile gardens offer less and less food ressources for insects. We should let some areas in our gardens 'untidy', so that many wild flowers can grow and bees find places to nest (in my other comment I also wrote about nesting aids).
There is also a list of bee-friendly plants (in German, but I think self-explaining with scientific names of the plants).