The largest variable affecting cross country soaring is the weather. You will need to forecast the soaring weather for the day of the flight.
The forecast will tell you the type of lift to expect. In thermal, so you will want to know the thermal height and strength to expect, and the workability of the thermals. On a ridge day the wind speed and direction at the surface are critical. And on a wave day the stability, wind speed and direction at altitude, and cloud cover are all important.
A week ahead, forecasts from MetVUW and MetService are useful. In particular, the MetService Mountain forecast is useful if you're near on of the areas it covers. The MetService three and five day rain and surface wind charts are also good. Windy.com has a lot of information up to a week ahead also.
Long range forecast soundings are available from NOAA. You will need the latitude and longitude of your site in decimal degrees, select the type of sounding you want, and enter the captcha code. It is worth playing around with the options here. These forecasts use UTC for time and pressure for altitude, so you'll want to become familiar with converting UTC into NZ time and hectopascals into feet.
A day in advance, those sources are still good, but now you can get the forecast surface pressure charts from Met Service, and the soaring forecast from RASP.
Light winds at low levels are more conducive to thermal activity. More than five or ten knots on the surface will reduce thermal development.
Wind in the thermal layer may be a problem if your glider is lower performance and you have to fly upwind.
Once the winds is getting to fifteen or twenty knots there will be ridge and wave to consider. Even if the wind is light on the surface, the upper winds can be strong enough for ridge and wave. Wave will affect the lower level thermal production.
A tephigram can be used to find out if the atmosphere is stable or unstable. Unstable air is likely to have potential for good thermal production. Stable air favours wave.
If there is an inversion the air above the inversion can have quite different wind to the air below. In that case there can be wind shear and turbulence at the inversion.
Cloud cover and cloudbase must be considered.
For thermal conditions no cloud means no markers, so finding thermals is more difficult. More cloud cover means less sun heating the ground, so total cloud cover isn't good for thermal either. Stratus cloud just blocks the sun, cutting the available energy. Cumulus clouds mark the lift, and having 50% cloud cover from cumulus is good. To much more will cut down the heating, reducing the thermal energy available. Cloud cover can be found from soundings, from RASP "Boundary Layer Cloud Cover" page and "Normalised Surface Sun" page, from satellite pictures, from looking out the window.
Cloudbase is critical for thermalling as it generally sets the top of the working band. You get to use from 1,000ft AGL or so, up to cloudbase as your working band when thermalling. The higher the cloudbase, the further you can go between thermals. Cloudbase can be calculated from a sounding, or from the RASP "Cumulus Cloudbase where CuPotential is more than 0" page.
For ridge soaring, cloud matters if it is low on the ridges. If cloudbase is above the ridge tops, you're good to go.
For wave soaring, the amount of cloud cover is very important. You need clear gaps to run in, and you don't want them closing in on you. Cloudbase is less important. Lower cloudbase typically means lower wave entry heights. Most of your task will be flown above cloudbase.
If the weather is changing during the soaring window you may need to fit your task to follow the weather.
A front moving through during the day may provide opportunities for soaring ahead of the front, then dead air behind.
Sea breeze activity may be common late in the day. You might task towards the sea early in the task, then inland later to stay ahead of the sea breeze.