If you use Turbo Basic XL (TBXL) for the Atari 8-bit, you will get more structures (REPEAT/UNTIL, IF/THEN/ELSE, etc.). However, if you want to use the built-in Atari BASIC (maybe you do not want to use OSRAM in the XL/XE systems which can cause compatibility issues), then you will have to make your own ELSE construct.
I was thinking about something like the following:
10 IF (this condition) THEN DIM X(10):GOTO 30
20 DIM X(30)
30 PRINT "Hello"
In this case, you will execute both statements on line 10 and skip the new DIM statement on line 20. I checked it out in Altirra (Atari 8-bit emulator) to make sure this worked in Atari BASIC. Here is the exact code I typed into Atari BASIC:
20 IF A=1 THEN PRINT "HELLO":GOTO 40
30 PRINT "Hello 2"
40 PRINT "I'M HERE"
When I run this simple program, I see HELLO and I'M HERE printed. If I change A to another value, it prints Hello 2 and I'M HERE. This should do what you want. I realize it is not as elegant as an IF/THEN/ELSE but, without using TBXL, I do not see an alternative.
As mikro mentioned, there are many more 8-bit programmers on AtariAge. As to a banner, The ST forum on AtariAge does receive more traffic than the 8-bit forum receives here. However, I have seen people on AA refer people here when they need more extensive ST help.