Using Declared Variable Within Select Statement?

here is what i am trying to do: im as using oracle 8 with sqltool

i have a Very large query. and i notice that many things are repeating. so i want to add them to a variable, instead of re-typing them. for example:

select SomeID from SomeTable;

i want SomeID to be put into a variable. but i still want to be able to get a normal select query at the end so that i can see the returned value:

i tried things like:
declare x number;
begin
set x=45454
select x from SomeTable;
end;

but could not get it to work.