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Perhaps difficult but far from the worst:|

The biggest problem here is the fact that many genera are externally similar. The genera are well-defined when using characters of male genitalia.

I definitely recomment Hennig's treatment of the Palaearctic fauna. It has good figures of male genitalia for almost all species. It's about 1000 pages but in my opinion worth photocopying (photocopying works for private use is legal at least here in Finland - for now). You can also buy it from the publisher, but it is *very* expensive, around 600?:o:(

My routine for identifying these flies is:
1. If you don't immediately know which genus, discard females.
2. Cut off end of abdomen and dissect male genitalia. For simple identification I don't use any complicated KOH softening baths. I just put the tip of abdomen in warm water for 10-15 min so it rehydrates, and then tease it apart with two needles. After examination I glue the genitalia on a small cardboard square and pin it together with the fly.
3. Grab 'the Hennig' and browse through the genitalia figures until you find a decent match.
4. Go back to genus & species-level keys and check if genitalia-ID match morpho-ID
5. Check against Hennig's detailed description

For a much more professional reply, contact Verner Michelsen. He lives in Denmark and certainly is the best source for literature recommendations etc.