There is no exact rule of thumb, but the best guide when dealing with chromatic harmony in a tonal context is to spell the chromatic notes as part of the harmony (or implied harmony) of the passage of music you are writing. In a passage in E minor, use a B flat if the harmony is built on a chord that uses B flat; G minor as a chromatic mediant would be a perfect usage of B flat instead of A sharp here. If the harmony is a chord that uses A sharp (imagine F sharp as a secondary dominant/applied dominant to B (Roman numerals: V7/V) as an example) then use the A sharp.
As to the question of whether you should use a C sharp or D flat, again it comes down to what the harmony is in exactly the same way. Think about the harmonic context of every chromatically altered pitch and write it according to the quality of the chord and how that chord functions.
Double sharps and double flats follow exactly the same principle. In the key of C sharp minor, again the use of certain applied dominants within the tonic key require double sharps (such as V7/II where C double sharp is the leading note to the root of chord II or V7/V where F double sharp is the leading note to the root of chord V) and other chromatic alterations (eg chromatic mediants) would need spellings that make them correctly appear as triads. In D flat major, one of the chromatic mediants you could come accross is a B double flat major triad, for example.
When it comes to non-chordal notes in a melodic line (passing notes, neighbour notes et al) you can employ chromaticism as well, although in this case it is always best to consider how intuitive it is to read. Usually the best way to judge this is to consider the contour of the line and the non-chord note's relationship with the next one (eg a non-chordal B flat descending to chordal A is much easier to read than a non-chordal A sharp descending to a chordal A natural). Avoid having to immediately cancel sharps and flats with naturals, if you can.
Personally, I find tonal music notoriously complex and convoluted when it comes to the notation of accidentals, it is very hard to understand for a beginner, but considering you are not a beginner then the only advice I can give is to have a look at the harmonic or melodic contexts of the music you are writing.