I'm working almost only on assembler(on assembler - for micros,on TP and C for PC), so i'm very serious asking this.

I)First of all - find documentation and read it.
II) Learn how to use libraries- how to write and implement. Do not work with single source file.
III) If Your assembler dos not provides warnings for wrong variable usage,
forget about BIT, DATA,XDATA . Simple use EQU and count/think about memory usage. Reason is - in some moment You will reuse some variables - once in some subrotine, next- in other. Assemblers usually do not provide such non-static variables. In that moment DATA , BIT and XDATA declarations will consume whole memory, if You aren't plane this situations . So good planning is essential here.
regards