Strategies for Scattergories
I think your techniques are sound if, first, you've read the categories so they are in the forefront of your subconscious when you consider the letters.

Scattergories is a bit similar to 'Boggle' (Parker Bros.) and 'dig-it!' (Cadaco), in that words one's opponents also use are not scored, but different from Scrabble (Hasbro) in that...well, everyone knows Scrabble.  But all these games primarily rely on vocabulary and imaginative thinking (neither of which is improved by your techniques).

Anecdote Alert: My Scrabble play dramatically improved once I got the Scrabble Player's Dictionary (SPD); which I read (past-tense) daily.  My other dictionaries didn't have many that are found in the SPD and I never would have known of them otherwise.  During gameplay, I let others use the SPD to find words and to challenge mine, I don't use it except for challenges.

I would suggest you do something to increase your Scattergory vocabulary.  Because of the aforementioned "duplicates don't count" mechanic the words/terms you'll want are those that don't normally come to mind.  

People tend to be predictable in how they think and what they think about.  Your goal should be to think differently, outside the mainstream (but not so far you can't defend a challenge), and increasing your Scattergory vocabulary will help do that.

Each day, pick a category (the ones that are most difficult for you), search the internet for a "list of ____", print it out, and scan it 2-3+ times during that day.  Don't try to memorize it, just say the words aloud, imagine what it looks like, think about which ones others may not know so well, and you're done.  Keep your list nearby during game play in case you're challenged.

For some categories, "lists" may not be practical because they would be too long. (My game is in storage so I'm going to guess...) 

IE. "Movie Stars"/"TV Stars", look up ie. "movie/TV stars of the 1940's"; 

"Boy's Name", look up "boys' names of [pick a country that uses the same/similar alphabet or translates well]...the Old Testament of the Bible works well if the other players are Christian (not Jewish), Greek and Gaelic names might work if one uses the English (not Greek/Gaelic) form, etc.

"Things in a kitchen", also break larger things down to their component parts but make sure those components aren't too ambiguous, ie. "pilot light", "heating coil", "drawer knob", "pot lid", "garbage bag", "dog dish", "sponge", "coffee filters".

If an item appears on a list you don't know, look it up, learn it, and not just for the game but for the sake of learning.

All I'll say about "having fun" and "being competitive" is that games are all about challenging oneself and enjoying the "striving to do better next time".  The other players are there for companionship, laughter, randomizing the game, and "fog of war" (FoW).