As an former RE Spark I would say, make sure you use your SLC when you're in every year. Ask your (clerk). And if you want to be a spark when you get out, start when you're in.
To give you context: In the army I did a lot of tours and as far as army sparkies go i was pretty experienced and knowledgeable, contrast this with my mate who was on the same class 1 as me never did any sparkying as an Engineer (You will find you are mostly a ground-worker my friend) But did saturdays with a guy for about 3 years. We left at the same time and he is on a 37k maintenance contract where I am having to build things up from a standing start.
3 Handy tips:
Take a Civi AM2 when you take your army one
Get a JIB card and if you get class 1 get the gold 1 on a one day cert while at chatham, The JIB centre is in Kent
Stay away from the Black Widow
2 other handy tips: I