Hi, there are a lot of things to consider when asking how to combat another character.
Some characters have a general weakness to others, such as melee vs ranged but in particular knowing which character you play would be great to know.
Here are my general tips to combating a good Tristana.
First, I play Nidalee but most of these tips will cross over to many different chars.
As I always go mid (90% of the time I'll kill whoever I face in mid) I face Tristana a LOT and a good one is beyond a hassle for any character, in particular the strategy that gets me and my playstyle is where she would flash (or just jump) behind me and knock me into her own tower, knowing this is a common tactic for her my gameplay changes for Tristana until I figure out how they themselves play her.
Most tristana's are aggressive which is the best way to play that character and as an aggressive player myself we tend to meet up for a lot of the battles especially early to mid game where those first few kills count.
In general I have gotten used to the flash and jump range so I usually stay out of it the real trouble is when they jump THEN flash but there's almost no way to stay outside of that range, my tip for any mid char is to take advantage of the middle section where she can't knock you far enough into her own tower, really put some damage into her there, an above poster mentioned she has low health and this is true, if you are playing a character who can pile on some damage or when you hit level 6 (like nidalee) your ulti lets you pile on the damage having her ALREADY low on health is a big plus.
For the start use healing potions, if your early gear setup doesn't let you afford healing potions I hate to say this but change it, if you CANNOT counter her range with another ranged char your almost certainly doomed to have her lower your hp over time until she can really do massive damage, which means when she hits level 6 you're dead.
Understanding how the character is played and what they are trying to achieve is a GREAT way to counter them, simply put when she gets a good idea for how much damage she does to you she's going to leap onto you, do a burst shot and knock you away from your tower, it doesn't matter which way just so long as you cant easily escape, to this end I have a strategy lined up for it (because it kills me some times as well).
Because Nid has a decent heal and I've made sure to keep trist on fairly low health as well it just comes down to whether I can survive to get the damage into her, for summoner spells I use Heal and Ignite, Tristana and a few other hero's are my reason for it.
When she uses her leap and blast combo I'm usually still in human form so I slam a heal on myself then use the summoner heal which changing into cat form and leaping at her while casting ignite and my other skills, usually though she takes away almost half my hp with that combo, because I know this I make sure to keep enough reactionary hp to heal myself.. some times I'm off but I just feel like baiting them into trying it :P
You can do that with other characters (my friend who plays poppy gets them pretty good like that too).
The real hassle is for the other lanes, Tristana can move in and our quick and her burst damage is so high I don't blame people for dying against her, what you should really be doing is listening to MIA calls and making sure you alert your team as well, she's an opportunistic killer, which means just like my fellow Nid and Shaco players her real objective to get a quick kill is to find some one already around half hp, most characters her level actually fare well enough against her... unless she's fed :P
Hide, use wards for warning (what no one else does that? :P) and be alert to MIA calls if your in the lanes.. abuse every one who doesn't make the MIA calls as well ^_^