Show off by tossing out a few Czech idioms
There are few things that will make you appear to speak more Czech than you really do and, right up there with cursing like a local but far more elegant (not to mention useful in any situation), one of the quickest ways is to learn and then use idioms.
Learn expressions of cultural significance
Idiomatic expressions are something that most foreigners learning any language do not regularly use and mixing them into your flow will make you instantly seem more linguistically adept.
Appear + be more advanced
Not only will your language appear stronger than it may (or may not) be, but you also get to learn some interesting vocabulary terms as you acquire the idiomatic phrases themselves.
Learn a couple, then a couple dozen
It is estimated that some 25,000 idioms exist in the English language (Source) but that is just plain too much to try and tackle. Instead, try learning just these 26 Czech idioms/expressions and incorporate them into your speaking as naturally as you possibly can.